LB 20-206 - Differential gene and isoform expression of HPG axis related genes during lamprey gonadogenesis

Background/Question/Methods:

Lampreys are extant representatives of ancient jawless vertebrates that are semelparous, have complex life histories, and are important models for understanding the evolution of vertebrate reproduction. Jawless fishes are the first extant lineage to possess a pituitary and are models for the development of the hypothalamic-pituitary-gonadal (HPG) axis signaling. This axis is the key regulator of vertebrate gonadogenesis, starting with the stimulation of hypothalamic gonadotropin-releasing hormone and pituitary gonadotropins in the brain along with the regulation of steroids and steroidogenic enzymes. The objective of this study was to identify transcripts associated with both sex and stage-specific gonadogenesis in sea lamprey including both differential gene and isoform expression. RNA-sequencing was performed on 28 gonad samples of male and female sea lamprey. A reference-guided de novo assembly pipeline was employed to align reads to the reference genome without the aid of a reference annotation to identify both known and novel genes. Following curation and annotation of the inferred genes, counts of genes and exons were obtained. A genome-wide differential gene expression (DGE) analysis was performed to compare differences in gene expression between sexes and between gonadal stages within sex, while differential exon usage (DEU) analyses were performed to identify sex-specific isoform usage.

Results/Conclusions:

The 28 gonad samples of male and female sea lamprey were sampled from individuals in early, mid, and late developmental stages, covering gonad development from early pre-differentiated gonads to upstream migrating adults. We identified 7091 genes at an FDR < 0.01 that showed sex-specific DGE; this included 2755 genes that were upregulated in females and 4355 upregulated in males. Further, we found 2896 genes that exhibit sex-specific DEU, of which 674 alternative-spliced genes showed 4-fold upregulation in females and 864 alternatively spliced genes showed upregulation in males. In gene-level analyses, we identified paralogues of genes involved in the HPG axis i.e., gth, gnrhr, cyp19, hsd3b, cyp17, cyp11, er, pr, arr which exhibited stage- and sex-specific DGE. Phylogenetic analysis of these genes revealed the phylogenetic relationship of these genes among 10 chordate species how and showed conserved function across different vertebrate lineage. Further, we found alternative-spliced variants of important steroidogenic enzyme genes and membrane receptors that play important roles in gonadal steroidogenesis. These data are integrated to generate a comparative model of genes underlying the HPG axis in sea lamprey versus those in later diverging vertebrates. This information would be helpful to design genetic control of sea lamprey in Great lakes.