Figure 1.

Clonal, hemiclonal and meroclonal reproductive modes of hybrid vertebrates (diploids: upper row; triploids: lower row) in comparison with sexual reproduction (upper left). Each column shows parental individuals, gametogenic pathways with germ cells, gametes and offspring genome composition (expanded from Lamatsch & Stöck [49] and Stöck et al. [50]. Letters and symbols: A, B: genomes of parental species that formed the hybrid taxon/form; when lacking additional modifying symbols, the genomes are usually inherited and transmitted clonally; symbols are explained overleaf, below the figure. Description of reproductive modes: Sexual reproduction (upper row): oogonia enter a normal meiosis, which results in recombined haploid ova; after fertilization by haploid sperm from conspecific males, diploid offspring with recombined maternal and paternal genomes form diploid male or female offspring. Hybridogenesis (upper row): hemiclonal reproductive mode, during which the genome of one parental species is eliminated from the germ cells [51–56]; the genome of the other parental species is either endoreplicated and undergoes meiosis without effects of recombination (e.g. diploid water frog, Pelophylax esculentus [55,57,58]) or gametogenesis is ameiotic (e.g. the livebearing fish Poeciliopsis monacha-lucida [51,56]). Diploid hybrid offspring emerge after fertilization of the haploid ovum by recombined allospecific sperm, usually from a parental, sexual species. Unnamed form of hybridogenesis (lower row): Clonal diploid eggs are possibly formed by the elimination of one of the double copied genomes while the remaining genomes undergo endoreplication followed by meiosis without effects of recombination (P. esculentus example [59,60]). Kleptogenesis: (upper and lower rows): occurs in unisexual salamanders, Ambystoma [61]. The genome of germ cells is endoreplicated, undergoes meiosis without effects of recombination resulting in diploid eggs (above) or triploid eggs (below) [62,63]. Ova may either be activated by allospecific sperm without karyogamy, i.e. like in gynogenesis (middle), be truly fertilized, leading to ploidy elevation of offspring (left), or sperm may in part replace one of the maternal genomes in the egg, followed by its partial elimination (right) [54]. Gynogenesis (upper and lower rows): formation of clonal gametes by an ameiotic process (example: Poecilia formosa, upper row [47,64]; example: Carassius langsdorfii, lower row [65]) or endoreplication (example: diploid Cobitis elongatoides-taenia, upper row [66,67]; example: triploid Cobitis 1elongatoides-2taenia, lower row [66–68]) of genomes in germ cells followed by meiosis without effects of recombination. Diploid gametes (upper row) or triploid gametes (lower row) are fertilized without karyogamy, followed by sperm genome elimination. Parthenogenesis (upper and lower row): Clonal gametes form via endoreplication of genomes in germ cells followed by meiosis without effects of recombination (example: Aspidoscelis tesselatus, upper row [69]; example: Aspidoscelis uniparens, lower row [70]). Alternatively, the genome of one parental species is extruded into the polar body and then fuses with the egg, restoring diploidy (Darevskia unisexualis) [71]. Eggs develop without sperm/fertilization. Meiotic (triploid) hybridogenesis (lower row): recombined haploid gamete formation after meiotic (example: Misgurnus anguillicaudatus [72]) or premeiotic elimination of a single copied genome (example: P. esculentus [55]); offspring are diploid. Pre-equalizing hybrid meiosis (lower row): occurs in allotriploid Batura toads (Bufo(tes) baturae) [50,73] and presumably also in related taxa (§5c(iii)). In females (left), a single copy genome (A) is separately endoreplicated and enters meiosis as ‘pseudo-bivalents' (box 1), along with bivalents of heterozygous chromosomes from another parental species (BB′). The formally tetraploid meiosis results in diploid gametes. Males (right) eliminate the single-copy clonal genome (A), while the two remaining genomes (B) undergo a normal meiosis (BB′) and form haploid recombined sperm. Batura toads present the only known gonochoristic vertebrate taxon with simultaneous Mendelian (BB′) and clonal (A) genome transmission. Fertilization results in triploid offspring of both sexes.