From the archives: Evolution of angiosperm self-incompatibility, genomic imprinting in wheat, and function of calnexin in the ER

January 2022: Evolution of angiosperm self-incompatibility

To prevent inbreeding, many angiosperms exhibit self-incompatibility (SI), which ensures that only pollen from a different plant can fertilize the ovules. There are different SI mechanisms, but all of them consist of tightly linked male and female expressed genes, which together form the S-locus. There are several outstanding questions about the evolutionary dynamics of SI. Zhao et al. (2022) dissected SI evolution in a study combining phylogenetics, transcriptomics, and functional analyses. They found a single origin for the widespread Type 1 SI, indicating this mechanism has been lost several times during evolution. They also showed that some plant families lost Type 1 SI, but subsequently regained SI through different mechanisms (Types 2–4; see Figure). The Type 1 SI S-locus consist of several S-locus F-box proteins (SLFs) linked to an S-RNAse. The pollen-expressed SLFs work together to detoxify maternal S-RNAse, but only if they are from a different haplotype. The authors asked whether an ancestral S-locus with a single SLF would be able to efficiently detoxify S-RNases. They performed functional analyses to show that SLFs are highly efficient at inactivating S-RNases, indicating that an ancestral S-locus containing an S-RNase linked to a single SLF would have allowed for successful outcrossing. Overall, this work revealed a highly dynamic picture of SI evolution and provides a molecular evolutionary framework for further SI studies.

Figure.

Distribution of self-incompatibility types in angiosperms. Figure credit: H. Zhao.

January 2018: Genomic imprinting in wheat

Genomic imprinting leads to mono-allelic expression in a parent-of-origin-dependent manner. It is found mostly in the triploid endosperm in plants and is thought to ensure correct gene expression levels for proper endosperm development. However, although imprinting has been found in many species, the overlap among imprinted genes between species seems limited. Yang et al. (2018) analyzed the conservation of imprinting between wheat species. Hexaploid wheat has undergone two allopolyploidization events, and its phylogenetic relationships with diploid and tetraploid species are well established. The authors first identified imprinted genes in different wheat species. To this end, they crossed different cultivars, performed RNA-seq on the endosperm, and used SNPs to identify allelic expression patterns. Genes that deviated from the expected 2Maternal:1Paternal ratio were defined as imprinted. To analyze the evolutionary conservation of imprinting, they compared imprinted genes between hexaploid wheat and the diploid and tetraploid species. They found that over half of the imprinted genes are conserved through the polyploidization events. In addition, homeologs created by the duplication events often are both imprinted. These results suggest that imprinted genes were largely conserved throughout the evolutionary history of wheat. Thereby, this study contributed to the ongoing debate on the conservation of imprinted genes (Montgomery and Berger, 2021).

January 1998: Function of calnexin in the ER

Acidification of endomembrane compartments is involved in many cellular processes, including osmoregulation and protein transport, and is therefore vital for normal cellular functioning. Vacuolar-type H1-translocating ATPases (V-ATPases) are responsible for this acidification, coupling ATP hydrolysis to proton transport. The V-ATPase complex consists of a peripheral domain (V1) that is responsible for ATP hydrolysis and a membrane-spanning proton channel (V0). It has been suggested that V0 and V1 are synthesized independently, but it is unclear how the two components are assembled. Previously, a 64kD protein had been copurified with the V-ATPase from roots (Ward and Sze, 1992). Li et al. (1998) identified this protein as calnexin, a molecular chaperone involved in proper protein folding and oligomerization at the ER. The authors showed that calnexin is a membrane protein, localized in the ER, associated with both partially and fully assembled V-ATPases. Furthermore, reciprocal co-immunoprecipitation showed that V-ATPase and calnexin indeed interact, although only a small amount of total V-ATPases is associated with calnexin. These findings suggested that calnexin acts as a molecular chaperone of V-ATPase assembly in the ER. Since then, it has been shown that calnexin also interacts with a variety of glycoproteins at the ER and is involved in the calnexin/calreticulin cycle that keeps glycoproteins at the ER until they reach their correct conformation (Aebi et al., 2010).