Gene expression during ovarian differentiation in parasitic and non-parasitic lampreys: Implications for fecundity and life history types

Lampreys diverged from the jawed vertebrate lineage approximately 500 million years ago. Lampreys undergo sex differentiation much later than most other vertebrates, and ovarian differentiation occurs several years before testicular differentiation. The genetic basis of lamprey sex differentiation is of particular interest both because of the phylogenetic importance of lampreys and because of their unusual pattern of sex differentiation. As well, differences between parasitic and non-parasitic lampreys may first become evident at ovarian differentiation. However, nothing is known about the genetic basis of ovarian differentiation in lampreys. This study examined potential differences in gene expression before, during, and after ovarian differentiation in parasitic chestnut lamprey Ichthyomyzon castaneus and non-parasitic northern brook lamprey Ichthyomyzon fossor. Eight target genes (17β-hydroxysteroid dehydrogenase, germ cell-less, estrogen receptor β, insulin-like growth factor 1 receptor, daz-associated protein 1, cytochrome c oxidase subunit III, Wilms' tumour suppressor protein 1, and dehydrocholesterol reductase 7) were examined. Northern brook lamprey displayed higher expression of cytochrome c oxidase subunit III, whereas chestnut lamprey displayed higher expression of insulin-like growth factor 1 receptor; these genes may be involved in apoptosis and oocyte growth, respectively. Presumptive male larvae had higher expression of Wilms' tumour suppressor protein 1, which may be involved in the undifferentiated gonad and/or later testicular development. Differentiated females had higher expression of 17β hydroxysteroid dehydrogenase and daz-associated protein 1, which may be involved in female development. This study is the first to identify genes that may be involved in ovarian differentiation and fecundity in lampreys.