Evolution of viviparity and polyembryony in Cyclostomata

The bryozoan order Cyclostomata (marine, colonial suspension-feeders) is unique among invertebrates in possessing an exceptionally rare reproductive pattern combining viviparity with matrotrophy (direct nutrient provisioning of an embryo during incubation) and polyembryony (development of multiple embryos from a single fertilized egg). This combination is only known in the vertebrate genus Dasypus (long-nosed armadillos). However, the mechanism of polyembryony in cyclostomes differs, thus making this bryozoan group an extraordinary model for the study of convergent evolution. The proposed project will aim to reconstruct the major stages in the evolution of the sexual reproduction in the Cyclostomata by focusing on comparative ultrastructural and microanatomical analysis of germ cell development, oogenesis, fertilization, placental analogues, embryonic development and larval structure that are almost non-studied and poorly understood. The new hypotheses on the ancestral type of reproduction, role of fertilization in the evolution of polyembryony, secondary loss of macrolecithal oogenesis and independent origin of placentation will be tested by comparing 20 species from twelve cyclostome families of six suborders representing all major cyclostome lineages. In particular the focus will be on the variety of nutritional and developmental adaptations: oocyte ultrastructure, the cytological mechanisms providing transfer of substances between maternal and embryonic tissues at the different stages of incubation as well as details of polyembryonic cleavage during formation of the secondary and tertiary embryos and larval development. The major innovative approach is an application of the data on sexual reproduction to the new molecular phylogeny that will be developed in the course of the project. In summary, beyond its broad relevance to the field of reproductive biology and evolution of reproductive modes in aquatic sessile organisms, the proposed research will provide an essential contribution to our understanding of the evolution of viviparity, extraembryonic nutrition and polyembryony in eumetazoans.

Extraembryonic nutrition or matrotrophy (direct nutrient provisioning of an embryo during gestation) and its most complex form placentation, is a masterpiece of Nature providing an increased offspring fitness and survival. Apart of mammals, it evolved many times in both, vertebrate and invertebrate animals, calling for comparative evolutionary analysis. Among aquatic invertebrates the phylum Bryozoa is unique in possessing extraembryonic nutrition and placental analogues in all major classes. The bryozoan order Cyclostomata is outstanding in having an exceptionally rare reproductive pattern combining viviparity with matrotrophy and polyembryony (development of multiple embryos from a single fertilized egg). Else, placental analogues have evolved at least 15 times in the order Cheilostomata, thus making bryozoans an extraordinary model for the study of convergent evolution of placentation. In the course of the project comparative ultrastructural and microanatomical analysis of oogenesis, placental nourishment and embryonic development was undertaken using species from two bryozoan clades above mentioned. In addition, data on transcriptomes of the species from all major cyclostome clades were obtained in co-operation with the Natural History Museum, London, to reconstruct the new phylogenetic tree and plot morphological data against it. The oocyte and ovary ultrastructure, cytological mechanisms providing transfer of substances between maternal and embryonic tissues at the different stages of incubation as well as details of polyembryonic cleavage during formation of the secondary embryos were studied using light and transmission electron microscopy. The particular focus was on incubation chamber (gonozooid) development and functioning to reveal patterns in formation and functioning of the placental analogues in different cyclostome and cheilostome species. Among the major discoveries are the new type of the placental structure (termed enveloping syncytial placenta) found in cyclostome bryozoans, complex tissue-like placental analogue in the cheilostome Celleporella hyalina, and formation of the oocytes in the pharynx wall of the cyclostome Cinctipora elegans. The former and the latter findings are of the exceptional importance pointing to the high evolutionary plasticity of sexual reproduction in Bryozoa. In general, the obtained data enabled to reveal major trends and reconstruct stages in the evolution of placentation in Bryozoa. In summary, beyond its broad relevance to the field of reproductive biology and evolution of reproductive modes, our project research provided an essential contribution to our understanding of the evolution of viviparity and extraembryonic nutrition in animals.