3.4 The expanded genes involved in the GnRH signaling pathway
The male O. bidens -specific expanded genes may be related to biological traits. Male O. bidens showed secondary sexual characteristics, such as nuptial organ on the surfaces of the head and extended-anal fins in the reproductive season. These secondary sexual characteristics are a result of the secretion of male hormones. KEGG enrichment analysis showed that the expanded genes were overrepresented in the GnRH signaling pathway (Figure 4). GnRH is the primary regulator of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) synthesis and secretion. The 78 expanded genes in the gonadotropin secretion pathway included LH and the glycoprotein hormone alpha chain (GSUa/CGa). The expanded genes p38MAPK and CaMK (CAMK2) were up to eight and seven copies, respectively (Table S6), suggesting an essential role for pituitary gonadotropes secreted from the anterior pituitary gland.
To uncover the mechanism of gene expansion, we examined the distribution of the expanded genes in the chromosomes of male O. bidens . The 78 expanded genes were dispersed in 29 chromosomes (Figure 5), accounting for 76.32% (29/38) of the whole chromosomes of male O. bidens . The top three chromosomes with expanded gene numbers were LG01 (seven genes), LG06 (six genes), and LG08 (five genes). A few expanded genes were clustered in one chromosome; for example, protein kinase C (PKC: PRKCB) with three genes was clustered in LG01 (EVM0000057, EVM0001656, EVM0005614), whereas part of PKC (PRKCB) was coupled in LG24 (EVM0001136, EVM0024807). This is true for PLA2 (EVM0020264 and EVM0034744) in LG06, and EVM0001701 and EVM0026993 in LG07, indicating that a piece of chromosome breaks off, resulting in gene expansions (Li and Li, 1987).