Genome-Wide Analysis Indicates Lineage-Specific Gene Loss during Papilionoideae Evolution

Abstract

Gene loss is the driving force for changes in genome and morphology; however, this particular evolutionary event has been poorly investigated in leguminous plants. Legumes (Fabaceae) have some lineage-specific and diagnostic characteristics that are distinct from other angiosperms. To understand the potential role of gene loss in the evolution of legumes, we compared six genome-sequenced legume species of Papilionoideae, the largest representative clade of Fabaceae, such as Glycine max, with 34 nonlegume plant species, such as Arabidopsis thaliana. The results showed that the putative orthologs of the 34 Arabidopsis genes belonging to 29 gene families were absent in these legume species but these were conserved in the sequenced nonlegume angiosperm lineages. Further evolutionary analyses indicated that the orthologs of these genes were almost completely lost in the Papillionoideae ancestors, thus designated as the legume lost genes (LLGs), and these underwent purifying selection in nonlegume plants. Most LLGs were functionally unknown. In Arabidopsis, two LLGs were well-known genes that played a role in plant immunity such as HARMLESS TO OZONE LAYER 1 and HOPZ-ACTIVATED RESISTANCE 1, and 16 additional LLGs were predicted to participate in plant–pathogen interactions in in silico expression and protein–protein interaction network analyses. Most of these LLGs’ orthologs in various plants were also found to be associated with biotic stress response, indicating the conserved role of these genes in plant defense. The evolutionary implication of LLGs during the development of the ability of symbiotic nitrogen fixation involving plant and bacterial interactions, which is a well-known characteristic of most legumes, is also discussed. Our work sheds light on the evolutionary implication of gene loss events in Papilionoideae evolution, as well as provides new insights into crop design to improve nitrogen fixation capacity.

Introduction

Genomic changes such as gene gain and loss events frequently occur during genome evolution (Domazet-Loso and Tautz 2003; Krylov et al. 2003). Genes specifically added to the genome of a species lineage are defined as “taxonomically restricted” genes (TRGs) or orphan genes and these have no significant sequence similarity to genes of other species lineages (Wilson et al. 2005). Comparative genomics has now demonstrated that TRGs are a universal feature of any genome (Khalturin et al. 2009; Tautz and Domazet-Loso 2011; Long et al. 2013; Arendsee et al. 2014) and play essential roles in the species evolution. Drosophila TRGs are involved in the evolution of lineage-specific ecological adaptations (Domazet-Loso and Tautz 2003), and Hydra TRGs play a role in the creation of phylum-specific novelties and in the innate defense system, and are thus also involved in species-specific adaptive processes (Khalturin et al. 2009). Lineage- or species-specific TRGs have also been identified in various plants such as in Arabidopsis thaliana (Lin et al. 2010), Oryza sativa (Campbell et al. 2007), Solanum spp. (Rensink et al. 2005), and legumes (Graham et al. 2004; Schmutz et al. 2010). Some TRGs are preferentially expressed in A. thaliana (Donoghue et al. 2011) and rice (Guo et al. 2007) in reaction to abiotic stresses, whereas O. sativa defense-responsive gene 10 (OsDR10), a rice tribe-specific gene, negatively regulates resistance to a broad spectrum of Xanthomonas oryzae pv. oryzae strains (Xiao et al. 2009). Therefore, TRGs are involved in the response to various stresses, thus contributing to adaptive evolution.

Genes can also be deleted from the genome during evolution. Some members in one gene family are often lost in certain lineages, but, in extreme cases, an entire gene family may be deleted from the genomes of certain lineages (Aravind et al. 2000; Demuth and Hahn 2009), creating lineage-specific lost genes. Gene loss and pseudogenization can lead to immediate loss of gene function, thus severely affecting major physiological processes of organisms. However, it may also open new developmental opportunities, confer a selective advantage, and serve as an engine for evolutionary change in bacterium and animals (Olson 1999; D’Souza et al. 2014; Gladieux et al. 2014). Loss of superfluous genes contributes to bacteria fitness (Koskiniemi et al. 2012) and is the driving force in the adaptation of parasites to eukaryotic cells (Merhej et al. 2009; Cisse et al. 2014; Sharma et al. 2014). The loss of a penicillin-binding protein may contribute to resistance to the cephalosporin drug, ceftazidime, in Burkholderia pseudomallei (Torok et al. 2012). Human-specific loss of a myosin heavy chain isoform expressed in the masticatory muscles has been linked to the weakening of human jaw muscles, which has been suggested to increase cranial capacity in humans (Stedman et al. 2004).

Gene losses are also involved in the evolutionary divergence of floral morphology in plants. The loss of an anthocyanin pathway enzyme is associated with the transition from blue to red floral pigmentation, thus resulting in phenotypic differences among species of the Andean Iochroma of the Solanaceae (Smith and Rausher 2011). The loss of lineage-specific MADS-box genes such as GLOBOSA or DEFICIENS is potentially associated with the evolutionary divergence of floral morphology during the radiation of the Euasterids I within core eudicots (Lee and Irish 2011). Heterotopic expression of a MADS-box gene 2-like from Physalis floridana (MPF2-like) is required for the fruiting calyx inflation trait called “Chinese lantern,” yet is physiologically known as inflated calyx syndrome (ICS) in Physalis (He and Saedler 2005), whereas loss of a copy of MPF2-like genes is involved in the loss of ICS in Tubocapsicum (Khan et al. 2009). Therefore, the evolution of lineage-specific gene loss can result in new morphological traits among species or genera.

The legume (Fabaceae) consisting of three clades, Papilionoideae, Caesalpinioideae, and Mimosaceae, includes important grain, pasture, and agroforestry species and is characterized by unusual flower structure, podded fruit, and the ability of most species to form nodules with rhizobia (De Faria et al. 1989). However, genetic variations that could distinguish legumes from nonlegumes have not been identified. In particular, nodule formation is a developmental process connecting plant and bacterial cell differentiation (Roux et al. 2014). Much of this process remains a mystery although a few symbiosis-related genes have been identified in legumes (Schauser et al. 1999; Catoira et al. 2000; Limpens et al. 2003). Genome sequencing of Glycine max and its comparison with distantly related species such as Populus trichocarpa has revealed specific gene gains in legumes (Schmutz et al. 2010). However, gene loss has not been evaluated in relation to the evolution of legumes. No genome of Caesalpinioideae and Mimosaceae has yet been sequenced, but the whole-genome sequencing of five additional legume species in Papilionoideae, the largest and most widely distributed clade of Fabaceae such as Lotus japonicus (Sato et al. 2008), Medicago truncatula (Young et al. 2011), Cajanus cajan (Varshney et al. 2011), Cicer arietinum (Varshney et al. 2013), and Phaseolus vulgaris (Schmutz et al. 2014) has allowed investigations on lineage-specific losses in Papilionoideae, thereby gaining insights into the adaptive role of gene loss in the entire legume family. In this study, we identified the legume lost genes (LLGs) through genome-wide comparative analyses of legume and nonlegume species. Thirty-four Arabidopsis genes had orthologs in nonlegume species but were not detected in legumes. Eighteen LLGs were directly or indirectly inferred to function in the plant–pathogen interaction in nonlegumes. Therefore, the loss of these genes might have partially contributed to genomic changes that were related to the evolution of symbiotic nitrogen fixation in legumes.

Materials and Methods

Sequence Availability

Whole genome-wide primary transcript sequences of G. max, P. vulgaris, M. truncatula, and 34 nonlegume species were downloaded from Phytozome v10 (http://www.phytozome.net, last accessed March 1, 2015). Sequences of L. japonicus were obtained from Kazusa DNA Research Institute (http://www.kazusa.or.jp/lotus, last accessed March 1, 2015), and those of C. cajan and C. arietinum were downloaded from the International Crops Research Institute for the Semi-Arid Tropics (http://www.icrisat.org, last accessed March 1, 2015). The version of each database is summarized in supplementary table S1, Supplementary Material online.

Identification of LLGs

To identify possible LLGs, we used all coding sequences (CDS) of A. thaliana to conduct Basic Local Alignment Search Tool (BLAST) analysis of sequences of earlier described legume species. The identification steps are presented in supplementary figure S1, Supplementary Material online. Putative LLGs were selected under the BLAST results, with an E-value cutoff of 1 × 10⁻⁵. To rule out Arabidopsis-specific genes, the putative LLGs were searched in three selected genomes of Vitis vinifera, Prunus persica, and P. trichocarpa (E-value = 1 × 10⁻¹⁰). Arabidopsis genes without any hits in six legume species but showing homologous sequences in all three nonlegume species were further verified at the protein level in the aforementioned nine species using bidirectional BLASTP (E-value = 1 × 10⁻⁴), and bidirectional best hits were defined as putative orthologs. When the best hit in a species of one putative LLG in Arabidopsis was also the best match for another putative LLG in Arabidopsis, the putative LLG’s ortholog (bidirectional best hit) was not considered to be lost from this species. When proteins with bidirectional best hits in three nonlegume species had hits in legumes with an E-value of <1 × 10⁻⁴ but did not show bidirectional best hits, these were defined as Group 1 LLGs, indicating that the legume species had lost orthologs of the LLGs. Protein sequences without any hits in the legume species but with bidirectional best hits in the three nonlegume species were defined as Group 2 LLGs. These BLAST results were further verified by orthoMCL v1.4 analysis (Li et al. 2003). Phylogenetic analyses were performed whenever a conflicting signal was observed among bidirectional BLAST and orthoMCL. Nucleotide sequences were aligned using the Clustal X v2.1 program with default parameters (Larkin et al. 2007). Alignments were optimized via manual adjustment, and partial sequences with poor alignment were excluded. Substitution saturation was tested using DAMBE v6.0.1 before phylogenetic analysis (Xia 2013). Unrooted maximum-likelihood trees were constructed using the PhyML v3.1 program using a generalized time-reversible model with 100 bootstrap resamplings (Guindon et al. 2010).

The LLGs’ orthologs were further characterized in 29 other angiosperm species and Selaginella moellendorffii by bidirectional BLASTP using protein sequences (E-value = 1 × 10⁻⁴). The phylogeny of the involved plant species was derived from Cogepedia (http://genomevolution.org/wiki/index.php/Sequenced_plant_genomes, last accessed March 1, 2015) and APG III (Angiosperm Phylogeny Group 2009). Ancestral character state reconstruction was performed with the Markov k-state 1 parameter model (Mk1) in Mesquite 3.03 (http://mesquiteproject.org, last accessed March 1, 2015). Information on the gene families of the LLGs was generated from the Phytozome v10 clusters at the angiosperm node. Gene ontology (GO) annotations of LLGs were derived from Blast2GO (Conesa et al. 2005) using the National Center for Biotechnology Information nonredundant database (September 30, 2015).

Identification of Conserved Genes in Angiosperms

The genes conserved in A. thaliana, V. vinifera, P. trichocarpa, P. persica, and all six legumes were used as controls throughout the work. To identify conserved genes, the primary CDS sequences of A. thaliana were subjected to BLAST analysis using the aforementioned three nonlegume and six legume species (E-value = 1 × 10⁻⁵), and then Arabidopsis genes showing homologs in the aforementioned nine species were subjected to BLASTP. The resulting protein-encoding genes showing bidirectional best hits in all nine species (E-value = 1 × 10⁻⁴) were designated as conserved genes.

Gene Structure Analysis

CDS length, intron number, and intron length covering the CDS of the identified LLGs in nonlegume species were obtained from gff3 profiles downloaded from Phytozome v10 and TAIR10 (http://www.arabidopsis.org, last accessed March 1, 2015).

In Silico Expression Prediction

The expression data of roots, seedlings, expanding leaves, stems, vegetative shoot meristems, whole inflorescences, flowers, and fruits of Arabidopsis were obtained from a previous work (Laubinger et al. 2008). Relative gene expression levels (Z-scores) in different tissues were calculated as previously described (Benedito et al. 2008). When the Z-score value of a given gene in a tissue was not <1.5, the gene was considered highly expressed in the tissue. Gene expression of the identified LLGs under various hormonal treatments and biotic stresses was based on a previous study (Ma and Bohnert 2007). The expression data of the identified LLGs’ orthologs in response to biotic stresses in tomato were taken from the Tomato Functional Genomics database (http://ted.bti.cornell.edu/, last accessed March 1, 2015), and related data of rice and grape were extracted from the Gene Expression Omnibus database (http://www.ncbi.nlm.nih.gov/projects/geo, last accessed March 1, 2015). Heat map of gene expression was performed using MeV 4.9.0 (http://www.tm4.org/mev.html, last accessed March 1, 2015).

Gene Coexpression and Enrichment Analyses

The genes coexpressed with the LLGs in A. thaliana were identified using the MAS5 algorithm in CressExpress v3.2 (http://cressexpress.org, last accessed March 1, 2015). The employed parameters were as follows: cutoff value for Kolmogorov–Smirnov quality-control statistic was 0.15, and R² threshold for pathway-level coexpression was set as 0.36. Genes coexpressed with each LLG were sorted by correlation index, and the top 50 (if <50, then all genes were used) were used in enrichment analysis. Protein sequences of genes coexpressed with each LLG were submitted to KOBAS 2.0 (http://kobas.cbi.pku.edu.cn, last accessed March 1, 2015) for the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and Biocyc enrichment analyses. GO enrichment analysis was performed on AGRIGO v1.2 (http://bioinfo.cau.edu.cn/agriGO, last accessed March 1, 2015), with the reference genome locus obtained from TAIR10. The P value of all enrichment analyses was set as 0.05.

Protein–Protein Interaction Prediction

The identified LLGs in Arabidopsis were submitted to the A. thaliana section of STRING 10 (http://string-db.org, last accessed March 1, 2015) to generate protein–protein interaction (PPI) networks.

Microcolinearity Analysis

To identify a syntenic block harboring a LLG between a legume species and a nonlegume species, we first determined the respectively 10 genes upstream and downstream of each LLG in the genome of Arabidopsis. OrthoMCL v1.4 (Li et al. 2003) was used to construct orthologous groups around LLGs across multiple plant taxa with default parameters. The schematic diagram of local genomic synteny was drawn manually.

Selection Test

Gene selection was evaluated by using the ω (ω = dN/dS; dN, nonsynonymous substitution rates; dS, synonymous substitution rates) that was calculated by the codeml program of PAML v4.8 under Models 0 (Yang 2007). The protein sequences were aligned using Clustal-omega v1.2.0, with default parameters (Sievers et al. 2011), and then the nucleotide sequences were aligned using a Perl module derived from ParaAT (Zhang et al. 2012). Poorly aligned regions were removed from each alignment using Gblocks v0.91 b (Castresana 2000) with the following parameters: type of sequence = codons, maximum number of contiguous nonconserved positions = 8, minimum length of a block = 10, and gap positions excluded from all sequences. After performing Gblocks, matrixes with nucleotide sites < 50 were discarded. The significance in ω difference was evaluated using the Kolmogorov–Smirnov test.

Results

LLGs Survey

To identify LLGs, we selected four nonlegume species and six legume species to initiate genome-wide comparisons (supplementary fig. S2, Supplementary Material online). We first used the Arabidopsis genome to probe the genomes of nine other plant species at the nucleotide level, and determined that 70 Arabidopsis genes had homologous sequences in three other nonlegume species, whereas their homologous sequences were not detected in the six legume species (supplementary table S2, Supplementary Material online), indicating a potential loss of these genes in legumes. We then verified these identified putative LLGs at the protein level. Using BLASTP, 34 of these Arabidopsis genes were found to have putative orthologous proteins in all three nonlegume species (V. vinifera, P. trichocarpa, and P. persica), whereas no putative orthologous proteins were detected in the six legume species (table 1 and supplementary table S2, Supplementary Material online). These genes were designated as LLGs and were further divided into two groups. In the legume species, 26 of the LLGs lost their orthologous genes but had homologous sequences and were thus designated as Group 1 LLGs. On the other hand, Group 2 LLGs included eight genes without any homologous sequences in various legume species (table 1).

Table 1.

LLGs

Query ID	Vv	Pt	Pp	Gm	Pv	Cc	Mt	Ca	Lj	Gene Symbol
AT1G09195.2 (1)	y (1)	y (1)	y (1)	N (0)	N (0)	N (0)	N (0)	N (0)	N (0)
AT1G35340.1 (1)	y (1)	y (1)	y (1)	N (0)	N (0)	N (0)	n (0)	n (0)	N (0)
AT1G64385.1 (1)	y (1)	y (2)	y (1)	N (0)	n (0)	N (0)	n (0)	N (0)	N (0)
AT2G43210.1 (1)	y (1)	y (2)	y (1)	n (0)	n (0)	n (0)	N (0)	N (0)	n (0)
AT2G43910.2 (2)a	y (0)	y (0)	y (0)	N (0)	N (0)	N (0)	N (0)	N (0)	N (0)	HOL1
AT2G43920.1 (2)a	y (0)	y (0)	y (0)	N (0)	N (0)	N (0)	N (0)	N (0)	N (0)	HOL2
AT2G43940.1 (1)a	y (2)	y (1)	y (1)	n (0)	n (0)	N (0)	N (0)	n (0)	N (0)	HOL3
AT4G14970.1 (1)	y (1)	y (1)	y (1)	N (0)	N (0)	N (0)	N (0)	N (0)	N (0)
AT4G22160.2 (1)	y (2)	y (3)	y (1)	N (0)	N (0)	N (0)	n (0)	n (0)	N (0)
AT4G29560.1 (1)	y (1)	y (1)	y (1)	N (0)	N (0)	N (0)	N (0)	N (0)	N (0)
AT5G44010.1 (1)	y (1)	y (1)	y (1)	N (0)	N (0)	N (0)	N (0)	N (0)	N (0)
AT5G49110.2 (1)	y (1)	y (1)	y (1)	N (0)	N (0)	N (0)	N (0)	N (0)	N (0)
AT5G65740.2 (1)	y (1)	y (1)	y (1)	N (0)	N (0)	N (0)	N (0)	N (0)	N (0)
AT1G13630.1 (1)	y (1)	y (1)	y (2)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT1G55580.1 (1)	y (1)	y (3)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)	LAS
AT1G55590.1 (1)	y (1)	y (1)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT1G68940.3 (3)d	y (2)	y (4)	y (2)	n (2)	n (1)	n (1)	n (1)	n (1)	n (1)
AT1G71120.1 (1)	y (1)	y (1)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT2G05810.1 (1)	y (1)	y (2)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT2G18520.1 (2)b	y (1)	y (2)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT4G36680.1 (2)b	y (1)	y (2)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT2G39100.1 (1)	y (1)	y (1)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT2G45530.1 (1)	y (1)	y (2)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT3G24515.1 (1)	y (1)	y (2)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT3G50950.2 (1)	y (1)	y (1)	y (2)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)	ZAR1
AT3G61210.1 (3)e	y (0)	y (0)	y (0)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT4G11670.1 (1)	y (1)	y (1)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT4G24340.1 (2)c	y (1)	y (3)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT4G24350.1 (2)c	y (1)	y (3)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT5G01015.1 (1)	y (1)	y (2)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT5G04840.1 (1)	y (1)	y (2)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT5G10830.1 (1)	y (2)	y (1)	y (2)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT5G12460.1 (1)	y (1)	y (2)	y (2)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)
AT5G66160.1 (1)	y (1)	y (1)	y (1)	n (0)	n (0)	n (0)	n (0)	n (0)	n (0)	RMR1

Vv, Vitis vinifera; Pt, Populus trichocarpa; Pp, Prunus persica; Gm, Glycine max; Pv, Phaseolus vulgaris; Cc, Cajanus cajan; Mt, Medicago truncatula; Ca, Cicer arietinum; Lj, Lotus japonicus. The symbol y indicates that the LLG had putative orthologs in this species, and n represents that the LLG had homologous protein sequences but no putative ortholog in this species. N indicates that the LLG had no homologous sequence in this species. Genes in bold indicate the Group 2 LLGs. The numbers in parenthesis represent number of genes in this species within the orthoMCL group containing LLG.

^aHOL1 and HOL2 are in one orthoMCL group; and HOL1, HOL2, and HOL3 belong to the HOL family.

^bAT2G18520 and AT4G36680 are in one orthoMCL group.

^cAT4G24340 and AT4G24350 are in one orthoMCL group.

^dAT1G68940 and two non-LLGs (AT1G20780 and AT1G76390) belong to one orthoMCL group.

^eAT3G61210.1 and two non-LLGs (AT1G55450 and AT3G54150) comprise one orthoMCL group.

We further performed orthoMCL analyses in the legume species, and 33 orthologs of the aforementioned LLGs were not detected (table 1), thus supporting the BLAST results. Nevertheless, a few inconsistencies were observed. No orthologs of AT1G68940 were detected in the legume species using BLAST, whereas in the orthoMCL analyses, AT1G68940 was determined to have possible orthologs in the legume species, and AT3G61210, AT2G43910, and AT2G43920 seemed to be specific to Arabidopsis (table 1). To assess these inconsistencies, phylogenetic analyses of related genes were performed (supplementary fig. S3, Supplementary Material online), and the results suggested that the orthologs of these genes were present in nonlegumes but absent from legumes. Thus, we ultimately identified 34 LLGs that belonged to 29 gene families in A. thaliana (supplementary table S3, Supplementary Material online). Group 2 LLGs (8 genes) belonged to seven families, and six LLGs in this group formed six single-copy gene family, except for AT2G43910 and AT2G43920 being homologs. In Group 1 LLGs (26 genes), ten were from single-copy gene families in Arabidopsis (supplementary table S3, Supplementary Material online). Multiple-copy gene families could also be lost during evolution such as the HARMLESS TO OZONE LAYER (HOL) family in Arabidopsis, which included AT2G43910 (HOL1), AT2G43920 (HOL2), and AT2G43940 (HOL3), and their orthologs were not detected in legumes (fig. 1 and supplementary table S3, Supplementary Material online).

Fig. 1.—

Local synteny around the HOL genes in legumes and nonlegume species. Species names are provided on the left. The block arrow on the horizontal line represents one open reading frame of a gene and its orientation. The yellow blocks are HOL family genes. The same color block arrows connected with the same color lines indicate the putative orthologs in different species. The gray block arrows are nonhomologous genes. The nomenclature of each putative gene is shown above the corresponding block arrow.

Evidence of Gene Loss from Microsynteny Analyses

To verify LLGs, we assessed for genomic microsynteny around LLGs in nonlegume species in relation to that of legumes. Although these plant species have increasing taxonomic distance and complicated genome structure due to duplications, losses, and segmental reshuffling during evolution, the stretches of syntenic chromosomal segments could be still identified. For example, in nonlegumes (A. thaliana, V. vinifera, P. trichocarpa, and P. persica), 1–3 HOL genes were clustered, and their downstream and upstream regions shared a few conserved genes, which was indicative of synteny (fig. 1). However, no HOL homologous sequences were detected in four legume species (G. max, P. vulgaris, C. cajan, and C. arietinum), although these were conserved in nonlegumes (fig. 1), thus indicating loss of HOL genes in these legumes. Altogether, 20 LLGs were found to maintain a relatively good local synteny among nonlegume species compared with legumes (supplementary table S4, Supplementary Material online). Thus, well maintenance of microsynteny verified the LLGs.

To understand the evolutionary implications of LLGs, we next investigated their evolution history in nonlegume plants.

The Evolution of LLGs in Nonlegume Angiosperms

Selection Pressure

We evaluated the selection pressure of these LLGs in the aforementioned four nonlegume plant species through calculating dN/dS (ω). We found that the ω values of LLGs was <0.35 (0.08 < ω < 0.33) (fig. 2) suggesting that these LLGs might have undergone purifying selection during nonlegume evolution. Moreover, there was no difference in selection pressures between Groups 1 and 2 LLGs (P = 0.15). We further identified 5,935 Arabidopsis genes having reciprocal best hits in the genomes of three nonlegumes and six legumes as conserved genes in angiosperms (supplementary table S5, Supplementary Material online). The ω of these genes in legumes ranged from 0.00010 to 0.65 (supplementary table S5, Supplementary Material online), while it ranged from 0.00064 to 0.48 in the four nonlegume species (indicated by black columns, fig. 2), indicating these conserved genes underwent purifying selection in both nonlegumes and legumes. However, the ω distribution of these conserved genes in nonlegumes was significantly different from that of LLGs (P = 7.81e−07; fig. 2). These results indicated that LLGs were generally conserved during nonlegume evolution, yet might have undergone a relatively relaxed purifying selection compared with conserved genes.

Fig. 2.—

Selection analyses of LLGs in nonlegumes. Selection was evaluated in Arabidopsis thaliana, Prunus persica, Populus trichocarpa, and Vitis vinifera. Yellow lines and cyan lines, respectively, indicate the dN/dS value of the Group 1 LLGs and Group 2 LLGs. The dN/dS distribution of the 5,935 conserved genes in these four plants is presented in black columns.

Gene Loss Patterns

We next investigated the loss pattern of LLGs, including additional 30 genome-sequenced plants that included 29 angiosperm species and S. moellendorffii (supplementary table S1, Supplementary Material online). The orthologs of LLGs in these plant species were identified (supplementary table S6, Supplementary Material online), and the presence–absence pattern of the orthologs of each LLG was mapped to the phylogenetic tree of the involved plants (fig. 3). The presence of the ortholog was indicated in blue. When no ortholog was detected, the presence of putative homologs was indicated in orange. In extreme cases, the absence of homologs was highlighted in gray. The number of species with LLG orthologs in nonlegumes ranged from 17 to 33, and on average, around 28 nonlegume species harbored LLG orthologs. These results again indicated that LLGs were conserved in most plants, whereas these were lost in legumes. Moreover, the absence of LLGs in nonlegumes seemed to be independent of their phylogeny (fig. 3), that is, Cucurbitales and Rosales are closely related to legumes, and the orthologs of 15 LLGs were not detected in C. sativus, whereas these were detected in P. persica, indicating that LLGs might have been randomly lost in a few nonlegume species during evolution. This is consistent with observations made in Poaceae (Poales) and Malpighiales. Poaceae are very different from legumes, and only six LLGs (AT1G09195, AT4G22160, AT2G39100, AT5G12460, AT1G71120, and AT3G50950) were absent from all investigated species of Poaceae (fig. 3), hinting that these might be lost in Poaceae, which was supported by ancestral state reconstruction (supplementary fig. S4, Supplementary Material online). A few LLGs were absent from Euphorbiaceae (Malpighiales) but existed in other species within Malpighiales. Some LLGs, such as AT5G44010, AT4G14970, and AT5G49110, may have undergone multiple, independent gene loss events throughout Angiospermae (fig. 3). Although the systematic loss of a few LLGs in other lineages could not be excluded currently (supplementary fig. S4, Supplementary Material online), the gene loss pattern of these LLGs in nonlegumes was similar to that of the previously identified 5,935 conserved genes in angiosperms (supplementary fig. S5, Supplementary Material online) but different from the evolutionary pattern of legume LLGs that was apparently specific and systematic (fig. 3). Ancestral state reconstruction further showed that the ancestors of Papillionoideae might have lost most of the LLG orthologs, whereas these LLGs’ homologs were found in the ancestors of angiosperms with a few exclusions such as AT1G35340 (fig. 3 and supplementary fig. S4, Supplementary Material online).

Fig. 3.—

LLG evolution in sequenced angiosperms. Gray represents species without any homologous sequence to LLGs. Orange indicates species with LLG homologs but not orthologs. Blue represents species with putative LLG orthologs. The black and gray stars indicate the ancestors of Papillionoideae and Angiospermae, respectively, and their ancestral states are represented at the bottom (for details, see supplementary fig. S4, Supplementary Material online). Forty plant species whose genomes have been sequenced are included, and their phylogeny was deduced from Cogepedia and APG III. Selaginella moellendorffii was used as outgroup.

Comparison of structural features (i.e., protein length, intron number, and average intron length) of conserved genes and LLGs in Arabidopsis (supplementary table S7, Supplementary Material online), as well as assessment of the evolution of LLG structural characteristics in sequenced species (supplementary figs. S6–S8, Supplementary Material online) did not indicate any distinct structural variations.

Functional Clues of the LLGs in Nonlegumes

We further explored the evolutionary implications of these LLGs by investigating the functional roles of these genes in nonlegumes. Literature search revealed that only six LLGs were functionally inferred (table 1), which included HOL1, HOL2, HOL3, LATERAL SUPPRESSOR (LAS), receptor homology-transmembrane-ring H2 domain Protein 1 (RMR1), and HOPZ-ACTIVATED RESISTANCE 1 (ZAR1), and GO annotation revealed that LLGs may involve different biological processes such as those that participate in DNA repair (GO: 0006281) of AT4G14970, AT5G49110, and AT5G65740 (supplementary table S2, Supplementary Material online). Some LLGs have been found to be associated with stress response such as HOL1, HOL2, and ZAR1 (GO: 0006952). AT5G10830 was annotated for respiratory burst involved in defense response (GO: 0002679), and AT3G61210 was annotated for response to ethylene and salt stress (GO: 0009723 and GO: 0009651). The functions of unknown LLGs in nonlegumes were next envisioned through in silico expression and PPI analyses in Arabidopsis because information on these LLGs in other angiosperms is limited.

Expression of LLGs in Arabidopsis

We investigated the expression of LLGs in different tissues of the plant model, Arabidopsis (see Materials and Methods). Besides lacking AT1G09195 expression, the remaining LLGs were differentially expressed in eight tissues, whereas some LLGs showed distinct tissue-specific expression patterns (supplementary fig. S9, Supplementary Material online). Eleven LLGs were highly expressed in roots such as LAS, HOL3, AT4G24340, and AT5G10830, whereas LLGs such as AT1G35340, AT3G50950, HOL2, AT4G24350, and AT1G71120 were highly expressed in expanding leaves, and LLGs AT1G64385, AT2G39100, and AT5G04840 were preferentially expressed in fruits (indicated by the red boxes, supplementary fig. S9, Supplementary Material online). On the other hand, no LLGs were highly expressed in Arabidopsis flowers (Z-score < 0.94).

We also investigated LLG expression in response to various biotic stimuli (e.g., hormones, elicitors, and pathogens). The transcript profiles of 30 LLGs were detected, whereas those of four LLGs such as AT1G09195, AT5G65740, AT1G64385, and HOL3 were not detected. In addition, 14 LLGs were involved in response to biotic treatments, including the two function-known genes, HOL1 and ZAR1 (fig. 4). The expression of HOL1 and its close homolog, HOL2, responded to hormone treatments such as abscisic acid (ABA), 1-aminocyclopropane-1-carboxylic acid (ACC), methyl jasmonate (MeJA), as well as to elicitors such as hairpin z (hrpz). In addition, challenging with bacterial pathogens such as Pseudomonas syringae pv. tomato DC3000 (PstDC3000), P. syringae pv. tomato avrRPM1 (Pstavrrpm1), and Botrytis cinerea (B. cinerea) resulted in a significant downregulation in expression of the two HOL genes (fig. 4 and supplementary fig. S10A, Supplementary Material online). ZAR1 also responded to these hormonal treatments and elicitors such as hrpz, and its expression was downregulated during PstDC3000 and Pstavrrpm1 treatments, but upregulated during P. syringae pv. tomato DC3000 hrcC (Psthrcc) and P. syringae pv. phaseolicola (Pstpsph) treatments (fig. 4 and supplementary fig. S10B, Supplementary Material online). Besides these three genes, Arabidopsis orthologs of 11 functionally unknown LLGs also showed differential expression (both upregulation and downregulation) in response to these treatments, which included AT1G35340, AT2G05810, AT2G18520, AT2G39100, AT3G61210, AT4G24340, AT4G24350, AT4G36680, AT5G01015, AT5G10830, and AT5G44010 (fig. 4), indicating that these genes are probably also involved in plant defense response in Arabidopsis. We further investigated the expression of these LLG orthologs in other plants whose transcriptomic variations challenging its own bacterial pathogens are publically available (supplementary table S8, Supplementary Material online), which showed that the orthologs of fourteen LLGs, such as AT1G35340 in rice, tomato, and grape also responded to various biotic stresses (table 2) suggesting that the role of each LLG may be conserved in other nonlegumes.

Fig. 4.—

Heat map of LLG expression under different stresses in Arabidopsis. Hormone treatments include ABA, ACC, and MeJA. Elicitors include CaCl₂ (Ca), glutathione S-transferase (GST), hrpz, GST-necrosis-inducing phytophtora Protein 1 (npp1), flagellin (flg), and lipopolysaccharide (lps). Bacterial stresses include Pseudomonas syringae pv. tomato DC3000 (PstDC3000), P. syringae pv. tomato avrRPM1 (Pstavrrpm1), P. syringae pv. tomato DC3000 hrcC (Psthrcc), P. syringae pv. phaseolicola (Pstpsph), Botrytis cinerea (B. cinerea), Erysiphe orontii (E. orontii), and Phytophthora infestans (P. infestans). The column above each treatment represents gene expression of different time points after treatment (for details, see supplementary fig. S10, Supplementary Material online).The color scale indicates the log2 values of expression change (treatments/control). Yellow indicates upregulation under treatments, and blue indicates downregulation under treatments. Green boxes indicate the log2 values of fold changes (treatments/control) > 1.0, and red boxes indicate the log2 values of fold changes (treatments/control) < −1.0. Gene names in red are Group 2 LLGs. AT2G43910/20 represents AT2G43910 (HOL1) and AT2G43920 (HOL2), and AT4G24340/50 represents AT4G24340 and AT4G24350 because one probe ID could detect the two closely related genes.

Table 2.

Summary of LLGs Involved in Plant Defense Response

LLGs	Arabidopsis			Rice	Tomato	Grape
	Function	Expression	PPI	Expression
AT1G35340.1		Y		Y	Y	Y
AT1G64385.1					Y
AT2G43910.2 (HOL1)	Y	Y
AT2G43920.1 (HOL2)		Y
AT4G14970.1			Y
AT4G29560.1				Y
AT5G44010.1		Y		Y
AT5G49110.2			Y
AT5G65740.2			Y
AT2G05810.1		Y		Y
AT2G18520.1		Y		Y
AT4G36680.1		Y		Y	Y
AT2G39100.1		Y
AT3G24515.1 (UBC37)			Y	Y
AT3G50950.2 (ZAR1)	Y	Y			Y	Y
AT3G61210.1		Y		Y		Y
AT4G11670.1					Y
AT4G24340.1		Y		Y		Y
AT4G24350.1		Y		Y		Y
AT5G01015.1		Y
AT5G10830.1		Y	Y	Y		Y

Y indicates that evidence for LLG involvement in defense response was detected. PPI, protein-protein interaction.

Enrichment Analysis of Genes Coexpressed with LLGs

Genes in the same pathway and genes that have related functions often exhibit similar expression patterns, which is why analysis of gene coexpression networks is a useful way of developing functional annotation (Usadel et al. 2009; Lin et al. 2010; Childs et al. 2011). To further explore the function of these LLGs, we performed coexpression analyses. A total of 21 LLG coexpressed gene sets were detected (R² > 0.36), which in turn were further subjected to functional enrichment analysis. The three coexpressed LLG groups included AT1G35340/AT5G01015, LAS/AT2G05810/AT4G29560, and AT2G18520/AT4G36680 (supplementary table S9, Supplementary Material online). Moreover, the genes coexpressed with LLGs were putatively involved in multiple fundamental biological processes such as DNA replication, ribosome biogenesis, protein processing, and secondary metabolites (supplementary table S10, Supplementary Material online). DNA replication (ath03030) was significantly enriched in the coexpressed genes of AT4G14970 (supplementary table S10, Supplementary Material online). Ribosome biogenesis (ath03008) and pyrimidine ribonucleotide biosynthetic process (GO: 0009220) were significantly enriched in the coexpression genes of AT2G18520 and AT4G36680, whereas protein processing in the endoplasmic reticulum (ath04141) and purine transport (GO: 0006863) were simultaneously enriched in the coexpressed genes of AT2G05810 and LAS (supplementary table S10, Supplementary Material online). The enriched KEGG pathway associated with secondary metabolites such as phenylpropanoid biosynthesis (ath00940) was significantly enriched in coexpression genes of AT5G10830 (supplementary table S10, Supplementary Material online). Furthermore, three LLGs (ZAR1, AT4G24340, and AT4G24350) were associated with plant defense (supplementary table S10, Supplementary Material online). For example, defense-response and incompatible interaction (GO: 0009814) and plant–pathogen interaction (ath04626) were enriched in the gene set that was coexpressed with ZAR1, whereas plant-type hypersensitive response (GO: 0010363) and jasmonic acid biosynthesis (PWY-735) were enriched in the gene set that was coexpressed with AT4G24340/AT4G24350 (supplementary table S10, Supplementary Material online).

PPIs Associated with LLGs

We further predicted PPI networks associated with the identified LLGs. Among all the LLGs examined, four PPI networks were detected (supplementary fig. S11, Supplementary Material online). Three networks involved in each of the two LLGs such as AT2G43210/AT2G05810, AT2G18520/AT4G36680, and AT4G24340/AT4G24350, whereas the largest PPI network was associated with four LLGs, AT3G24515, AT4G14970, AT5G49110, and AT5G65740 (supplementary fig. S11, Supplementary Material online), suggesting that these played a role in protein ubiquitination because AT3G24515 encoded the putative ubiquitin-conjugating enzyme 37 (UBC37).

We also observed that LLGs or LLG PPI networks interacted with various non-LLG proteins that were involved in a wide range of biological processes such as DNA repair, cell division, protein processing, and plant defense (supplementary tables S11 and S12, Supplementary Material online). The LLG (AT2G43210)-interacting proteins included Arabidopsis Cell Division Cycle 48B (AtCDC48B), AtCDC48C, AtCDC48, Radiation-Sensitive 23B (RAD23B), RAD23C, and RAD23D (supplementary table S11, Supplementary Material online), which was suggestive of its involvement in cell division. On the other hand, LLG AT5G10830 appeared to be associated with factors such as BONZAI association Protein 1 (Yang et al. 2007; supplementary table S11, Supplementary Material online), which suggested that it might play a role in plant immunity. Interestingly, the largest PPI network of the four LLGs interacted with ten non-LLG proteins (fig. 5). Eight of these proteins such as ataxia-telangiectasia mutated (ATM), ataxia telangiectasia-mutated and Rad3-related protein (ATR), Nijmegen breakage Syndrome 1 (NBS1), breast cancer 2-like B (BRCA2B), BRCA2 (IV), meiotic recombination 11 (MRE11), ultraviolet-hypersensitive 1 (UVH1), and Fanconi/Fancd2-associated Nuclease I (FAN1) participated in DNA repair, whereas two proteins were from the HECT ubiquitin-protein ligase (UPL) family of proteins such as AT4G38600 (UPL3) and AT5G02880 (UPL4), thus indicating that these LLG proteins is likely involved in ubiquitination (fig. 5 and supplementary table S12, Supplementary Material online).

Fig. 5.—

The largest PPI network associated with LLGs. Each node represents a protein. The four nodes covered by a green circle indicate the four LLGs. The colorful connecting lines represent the types of evidence supporting each association: coexpression (dark brown), experiments (pink), databases (cyan), homology (violet), and text mining (light green).

Other possible developmental roles of these LLGs genes could not be ruled out in plants but extensive data mining suggested that LLGs and their orthologs in nonlegumes are associated primarily with plant defense response (table 2).

Discussion

Gene loss has been investigated in the past decade (Aravind et al. 2000; Moran 2002; Krylov et al. 2003; Wang et al. 2006); however, its importance has only recently attracted attention. Gene loss probably affects organisms to a greater extent than do most amino acid substitutions, thus serving as one of the main drivers in the evolution of gene families, morphological diversity, and adaptation (Lee and Irish 2011; Smith and Rausher 2011; Koskiniemi et al. 2012; De Smet et al. 2013; Dakovic et al. 2014), as well as in organogenesis and speciation (Scannell et al. 2006; Castro et al. 2013). However, gene loss during legume evolution has not been extensively investigated. In this study, we evaluated gene loss events that might have occurred during the evolution of Papilionoideae at the genome-level, and identified 34 LLGs that were lost in a legume-specific manner (fig. 3 and table 1). Altogether 21 LLGs and orthologs in nonlegume species were determined to be associated with plant defense systems (table 2). Therefore, adaptive evolution of Papilionoideae might be implicated in the evolution of these LLGs.

LLGs Are Largely Involved in Plant Defense Response in Nonlegumes

The identified LLGs belonged to multiple gene families, and most of these were not functionally inferred. Based on literature search, gene expression analysis, and PPI prediction, we determined that LLGs might have played diverse roles in nonlegumes. LLG AT5G66160, which encodes the receptor homology region transmembrane domain ring H2 motif Protein 1 (AtRMR1), functions as the sorting receptor of phaseolin, thus facilitating in trafficking protein molecules to its corresponding storage vacuole (Park et al. 2005). LLG AT1G55580, which encodes LAS, plays a key regulatory role in the formation of lateral shoots during the vegetative development of tomato (Schumacher et al. 1999), Arabidopsis (Greb et al. 2003), and cucumber (Yuan et al. 2010). On the other hand, LLG AT2G43210 is possibly involved in cell division because most of its protein partners play essential roles in the cell cycle (supplementary table S11, Supplementary Material online; Park et al. 2008; Farmer et al. 2010). We also determined that a substantial amount of LLGs were involved in biotic stress responses (table 2). HOL1 is involved in the defense response to pathogens in Arabidopsis (Nagatoshi and Nakamura 2009), whereas ZAR1 is responsible for the recognition of the P. syringae Type III secreted effector HopZ1a, which attenuates HopZ1a virulence (Lewis et al. 2010). Similar to the two well-known defense-response genes, 12 LLGs were determined to respond to various biotic stresses in Arabidopsis (fig. 4 and table 2), and ten of the 14 LLGs were also detected in either rice, tomato, or grape (table 2), indicating that these might also participate in the defense response. Moreover, genes sharing a common role or function in a particular pathway were coexpressed. We observed that genes coexpressed with LLGs ZAR1, AT4G24340, and AT4G24350 were enriched in the regulation of plant defense response (supplementary table S10, Supplementary Material online).

The proteins in a PPI network are also probably involved in the same functional pathway (Lin et al. 2015; Zhang et al. 2015). Our PPI network prediction provides substantial informative functional clues for some LLGs. Notably, the largest PPI network that associated four LLGs AT3G24515 (UBC37), AT5G49110, AT5G65740, and AT4G14970 interacted with ATM, ATR, NBS1, BRCA2B, BRCA2(IV), MRE11, UVH1, and FAN1. Arabidopsis ATM, ATR, and NBS1 were involved in double-strand breaks of meiosis (Garcia et al. 2003; Waterworth et al. 2007; Culligan and Britt 2008), and BRCA2B and BRCA2(IV) are important for both DNA break repair and homologous recombination in somatic or meiotic cells (Abe et al. 2009; Seeliger et al. 2012). MRE11 (AT5G54260) plays a role in the early stages of MRE (Bleuyard et al. 2004; Puizina et al. 2004). UVH1 (AT5G41150), also known as AtRAD1, is a homolog of the yeast repair endonuclease RAD1, and is involved in nucleotide excision repair and telomere stability (Fidantsef et al. 2000; Vannier et al. 2009). FAN1 (AT1G48360) is involved in DNA crosslink repair (Herrmann et al. 2015). These observations indicate that these LLGs were presumably involved in DNA repair. Increased somatic recombination was observed in plants subjected to pathogen stress (Lucht et al. 2002). Furthermore, the DNA damage repair proteins, BRCA2 and RAD51, are involved in the regulation of plant defense gene expression (Choi et al. 2001; Durrant et al. 2007; Wang et al. 2010; Song et al. 2011), indicating that the LLGs associated with DNA repair might also be involved in plant responses to microbial pathogens. Furthermore, AT4G38600 (UPL3) and AT5G02880 (UPL4), which were observed in the largest LLGs’ PPI network, exert a role in ubiquitination system (Downes et al. 2003), and ubiquitination is required in plant immunity for the degradation of invading proteins (Trujillo and Shirasu 2010). Therefore, our multiple lines of evidence suggest that around 21 LLGs and orthologs are directly or indirectly involved in plant defense responses.

The Evolution of LLGs in Angiosperms

A gene that is continuously maintained in the genome indicates that it plays an essential role in viability (Krylov et al. 2003). In contrast, a gene without extensive and essential biological functions could be lost during evolution. Therefore, selection could be a significant driving force of gene loss (Koskiniemi et al. 2012). Because there are no whole-genome sequences available for the other two clades of Fabaceae, this study determined that LLGs specifically originated from Papilionoideae during legume evolution. On the other hand, nonlegume angiosperm species apparently had conserved these LLGs in its genome, which subsequently underwent negative selection. Single-copy genes often exhibit higher sequence conservation than nonsingle copy genes (De Smet et al. 2013). In line with this observation, 16 LLGs were single-copy families in Arabidopsis, whereas these maintained a low number of copies in most nonlegume species (supplementary table S3, Supplementary Material online). Therefore, LLGs might have originated from a direct gene deletion from the genome of a legume ancestor, instead of sequence divergence that mainly occurred when a gene is subjected to positive selection. The protein length and exon number of orphan genes, also called TGSs, are significantly different from those of nonorphan genes (Domazet-Loso and Tautz 2003). However, this study determined that the structural evolution of LLGs, lineage-specific lost genes, did not play a role in the emergence of these LLGs during plant evolution. Therefore, the major factors that drove the lineage-specific loss of these LLGs remain unclear. Nevertheless, most LLGs originated from ancestral legumes. Gene loss has been considered as a common and advantageous response during the genome evolution of living organisms (Wang et al. 2006). However, the role of LLGs in legume evolution requires more extensive investigations. Nonetheless, in the light of the putative role of LLGs in response to biotic stresses, we speculate that the loss of these genes plays a beneficial and adaptive role in the evolution of legumes.

Evolutionary Implication of LLGs

Root nodule is specialized organ of legumes, and nodule formation is initiated through the molecular cross-talk between a bacterium and a plant, thus involving a complex and precise interplay between host and symbiont, and shifting the intracellular signaling from defense response to symbiosis (Beck et al. 2008; Nakagawa et al. 2011). Several symbiosis-related genes have been identified in legumes, and their mutants show various defects in the nodule formation (Schauser et al. 1999; Catoira et al. 2000; Limpens et al. 2003). These symbiosis-related genes have orthologs in nonlegumes (Zhu et al. 2006; Zhang et al. 2009). Therefore, the lineage-specific gain or loss of certain genes is likely required in the development of the legume nodulation pathway.

Legume-specific gene families have been identified (Silverstein et al. 2006; Schmutz et al. 2010), and some of these show root- and/or nodule-specific expression (Severin et al. 2010), thus indicating the potential role of lineage-specific gene gain in the formation or maintenance of symbiosis. As compensation, we exploited the possible role of lineage-specific gene loss in the evolution of legumes. Eighteen LLGs in Arabidopsis were determined to participate in defense response (table 2) such as HOL1, HOL2, and ZAR1. In particular, four LLGs in a PPI network (AT3G24515, AT4G14970, AT5G49110, and AT5G65740) were apparently involved in plant–bacterial interactions. Moreover, some LLGs associated with defense responses were highly expressed in roots such as AT2G18520, AT4G36680, AT4G24340, AT3G24515 (UBC37), and AT5G10830 (table 2). LAS was also upregulated in roots but was apparently not associated with nodulation, whereas LAS was predicted to interact with carotenoid cleavage Dioxygenase 7 (supplementary table S11, Supplementary Material online), which is the ortholog in L. japonicus that controls determinate nodulation (Liu et al. 2013), thus suggesting that the root-expressed LAS might also be involved in nodulation. Therefore, LLGs could largely contribute to the improvement of compatibility between legume and rhizobia, thereby facilitating the establishment of reciprocal symbiosis.

In summary, through a genome-wide comparison, we identified a set of LLGs. The mechanisms and driving forces of LLG losses remain elusive; nonetheless, evolutionary loss of certain genes that are involved in plant immunity may provide new insights into elucidating the mechanisms underlying symbiotic nitrogen fixation. This work, for the first time, sheds light on the evolutionary implications of gene loss events in the evolution of Papilionoideae. Whether these findings can be generalized across the entire legume family requires further investigations. Engineering nitrogen-fixed genes in crops is essential for sustainable food production, and the results of this study thus also suggest that knocking out certain LLGs should also be considered in such kind of crop design.