The genomes of the livebearing fish species Poeciliopsis retropinna and Poeciliopsis turrubarensis reflect differences in reproductive strategy

Species from the livebearing fish genus Poeciliopsis are interesting models for the evolution of the placenta. Within this genus, the placenta has evolved independently three times. In mammals, it has been shown that the evolution of the placenta is accompanied by rapid evolution of genes involved in processes that regulate mother-offspring interaction, such as placenta formation, embryonic development and nutrient transfer to offspring. However, it is unclear whether this is also the case for the much more recently evolved placenta in the genus Poeciliopsis. To investigate this question, we sequence and assemble the genomes of two Poeciliopsis species: the placental Poeciliopsis retropinna, and the non-placental Poeciliopsis turrubarensis. For this, we use two third-generation sequencing techniques: PacBio sequencing for P. retropinna, and 10X genomics chromium technology for P. turrubarensis. The final assemblies comprise 622 Mb for P. retropinna with a scaffold N50 of 21.6 Mb, and 597 Mb for P. turrubarensis with a scaffold N50 of 4.2 Mb. Using the contiguity of these assemblies to our advantage, we assess gene duplications between the two genomes. Additionally, we perform a genome-wide scan for genes evolving under positive selection in both genomes. We report a number of distinct gene duplications in both species, some of which are involved in spermatogenesis, as well as nutrient provisioning to offspring. Additionally, we find several genes that evolve under positive selection. Based on our results, we can compare the genomic consequences of poeciliid placenta evolution with those of mammalian placenta evolution.