Unexpected Oogenic Pathways for the Triploid Fish Chrosomus eos-neogaeus

Triploid vertebrates from unisexual complexes often perpetuate themselves asexually. In the fish Chrosomus eos×eos-neogaeus, triploids are continuously produced by diploid hybrids. However, they are not expected to perpetuate since C. eos are their only known progeny. This study aims to investigate the oogenesis of these triploid hybrids through experimental crosses. A total of 337 larvae from 12 female triploids and 3 2n/3n mosaics fertilized with C. eos sperm were genetically characterized. The detection of C. eos as progeny of triploid hybrids confirmed the occurrence of a pathway similar to meiotic hybridogenesis but only for half of the tripoids. The presence of tetraploid offspring for all these females revealed the formation of unreduced triploid eggs as a probable failure of meiotic hybridogenesis. The remaining female triploids and all mosaics produced diploid and triploid hybrids. Triploids excluded the haplome from paternal leakage and produced eggs with the diploid hybrid genome through an ameiotic hybridogenesis. Both types of hybridogenesis occurred in a mutually exclusive manner. This leads us to consider 2 hypothetical scenarios: First, any female triploids can perform either type of hybridogenesis, allowing the long-term persistence of triploid hybrids by a fraction of the population. Alternatively, ameiotic hybridogenesis occurs in triploids of the first generation (from diploid mothers), while meiotic hybridogenesis occurs in triploids of the second generation (from triploid mothers); triploid hybrids then are not perpetuating lineages. The population dynamics of the C. eos-neogaeus complex appears a step more complicated than previously expected.