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).