Is There Conservation of Vitellogenins in a Placentotrophic Lizard?

Abstract

In vertebrates, the provision of nutrients to developing embryos varies widely, ranging from yolk-dependent strategies to highly specialized forms of placental nourishment. Vitellogenins (VTGs) are essential proteins for egg yolk formation in oviparous and lecithotrophic species. In contrast, in eutherian mammals, the loss of VTGs is associated with the evolution of matrotrophy (placentotrophy and lactation), where maternal nutrition via the placenta replaces the need for large yolk reserves during embryonic development. Marisora sp., a placentotrophic viviparous lizard with the most complex placenta known in reptiles, exhibits truncated vitellogenesis, resulting in the production of microlecithal eggs. This study investigated the presence of VTGs in Marisora sp. using RNA-seq from the liver and ovary at previtellogenesis and vitellogenesis stages. No corresponding annotations for VTGs were found. This absence may be associated with the placentotrophic nutrition of the embryo, suggesting modifications in lipid production and transport to the ovarian follicles. Apolipoprotein B (ApoB) and microsomal triglyceride transfer proteins (MTP) were identified, which are closely related to VTGs and could fulfill their function, especially ApoB, which is involved in yolk formation in lecithotrophic species in which VTGs are absent. The absence of VTGs in the Marisora sp. transcriptome represents a key discovery in the evolution of obligate placentotrophic viviparity in reptiles, highlighting convergent traits with mammals. Genomic studies are required to determine if changes in VTG genes prevent or modify their expression, and proteomic studies are needed to fully understand the role of other lipid transport proteins in the preovulatory ovarian follicles of these lizards.