Maternal factors and the evolution of developmental mode: Evolution of oogenesis in Heliocidaris erythrogramma

Evolutionary change in developmental mode in sea urchins is closely tied to an increase in maternal provisioning. We examined the oogenic modification s involved in production of a large egg by comparison of oogenesis in conge neric sea urchins with markedly different sized oocytes and divergent modes of development. Heliocidaris tuberculata has small eggs (95 mu m diameter) and the ancestral mode of development through feeding larvae, whereas H. e rythrogamma has large eggs (430 mu m diameter) and highly modified non-feed ing lecithotrophic larvae. Production of a large egg in H. erythrogramma in volved both conserved and divergent mechanisms. The pattern and level of vi tellogenin gene expression is similar in the two species. Vitellogenin proc essing is also similar with the gonads of both species incorporating yolk p rotein from coelomic and hemal stores into nutritive cells with subsequent transfer of this protein into yolk granules in the developing vitellogenic oocyte. Immunocytology of the eggs of both Heliocidaris species indicates t hey incorporate similar levels of yolk protein. However, H. erythrogramma h as evolved a highly divergent second phase of oogenesis characterised by ma ssive deposition of non-vitellogenic material including additional maternal protein and lipid. Maternal provisioning in H. erythrogramma exhibits reca pitulation of the ancestral vitellogenic program followed by a novel oogeni c phase with hypertrophy of the lipogenic program being a major contributor to the increase in egg size.