Embryonic Lethality, Juvenile Growth Variation, and Adult Sterility Correlate With Phylogenetic Distance of Danionin Hybrids.

Hybrid incompatibility, which plays a pivotal role in speciation, is expected to correlate with greater phylogenetic distance. Here, we investigate the fitness of interspecies hybrids within the Danionin subfamily, which includes the model species, Danio rerio, and its relatives - Danio kyathit, Danio albolineatus, Danio margaritatus, and Devario aequipinnatus. We generated hybrids through in vitro fertilization, using Danio rerio as the maternal species, with normal fertilization rates showing no incompatibilities in sperm-egg interactions within these two genera. Generally, all hybrids exhibit normal patterns and timelines in early developmental transitions, from cleavage stages to the initiation of epiboly, although inter-genera Danio-Devario hybrids subsequently exhibit fully penetrant embryonic lethality. Intra-genus Danio hybrids, on the other hand, can survive through embryogenesis and into adulthood. However, rates of survival during these stages diminish according to phylogenetic distance, with increasing early lethality in hybrids from more distantly related species. Additionally, Danio hybrids exhibit increased growth rate variability during juvenile stages. All Danio hybrids have reduced testes sizes, sperm counts, and sperm viabilities, with sperm displaying defects in flagellum formation and integrity. Adult male intra-genus hybrids are invariably sterile, except in the case of Danio rerio hybrids with the closely related Danio kyathit, which produced a backcrossed F2 generation that did not survive juvenile stages. Our studies highlight a loss of hybrid compatibility at various life stages in the Danio and Devario genera, based on deleterious effects and reduced developmental robustness, emphasizing a correlation between the severity of incompatibility outcomes and the degree of phylogenetic relatedness.