Pre- and post-fertilization maternal provisioning in livebearing fish species and their hybrids (Poeciliidae: Poeciliopsis)
*Correspondence author. E-mail: firstname.lastname@example.org
- 1Viviparous organisms vary greatly in their level of post-fertilization maternal provisioning and extensive provisioning (including very complex placentas) has evolved dozens of times in vertebrates. Through the use of species representing transitional states we aim to explore and develop empirically supported hypotheses that explain how such a complex trait has evolved.
- 2In this study, we investigated the life histories and patterns of post-fertilization maternal provisioning in naturally occurring, all-female, hybrids between Poeciliopsis monacha, a viviparous species that produces large eggs and nourishes its embryos via stored yolk (lecithotrophy) and P. lucida, which produces small eggs and nourishes its embryos after fertilization via a follicular placenta (matrotrophy). Hybrids can be diploid or triploid, leading to different levels of gene dosage from the parental species.
- 3We found that the extent of superfetation and average brood size of the hybrids did not differ significantly from their sexual parental species even though the latter was much lower in the hybrids. Moreover, the allodiploid, P. monacha-lucida, has intermediate sized eggs, whereas the allotriploid, P. monacha-monacha-lucida, produces eggs of similar size to those of P. monacha. The degree of post-fertilization maternal provisioning was unrelated to genomic dosages, with wild-caught hybrids being lecithotrophic, as seen in the maternal species (P. monacha).
- 4Our results with naturally occurring P. monacha-lucida hybrids differed from those obtained with laboratory-synthesized hybrid clones, which were either lecithotrophic or weakly matrotrophic. Consequently, we propose two non-mutually exclusive hypotheses to explain why lecithotrophic clones might be favoured in the wild. One hypothesis suggests that lecithotrophic hybrids minimize competition with the matrotrophic paternal sexual species from which they require sperm to reproduce. Alternatively, we propose that natural selection has eliminated matrotrophic hybrids that suffer from intergenomic conflict in the degree of post-fertilization maternal provisioning.
- 5We have thus identified an empirical system and developed testable hypotheses that will help elucidate the selective pressures that caused the evolution of post-fertilization maternal provisioning and potentially provide an opportunity to explore how epigenetic factors (e.g., intergenomic conflict) might influence this complex trait.