4.4 Candidate genes within genomic regions of putative sweeps
The regions with potential selective sweeps for invasive diploid populations of C. auratus were widely detected across the genome. The SNPs within these regions were annotated to 191 candidate genes with enriched GO terms mainly related to mannosidase activity and embryo development (Table S4). The biological significance of selection in genes related to mannosidase are not immediately obvious, although in humans mannosidase deficiency can cause disease, such as congenital dyserythropoietic anemia (Moremen, 2002). The selective sweeps related to genes involved in embryo development may be attributed to an advancement of spawning time in Tibet (Liu et al., 2015a) and subsequent exposure of embryos to the cold waters. The water temperature during reproduction period is about 20 °C in the native range (e.g. the mid and lower Yangtze River) and is below 10 °C in the invasive range (Huo et al., 2013; Zhang et al., 2016). Low temperature exposure can increase the mortality and frequency of abnormal larvae in C. auratus(Wiegand et al., 1989), resulting in individual selection during embryonic development.
For triploids, selection occurs at the whole individual level owing to the asexual gynogenesis and would be expected to be low, as numbers of mitogenome haplotypes and genetic diversity in invasive populations were not reduced as compared to the source populations. Hence, it is doubtful whether signals of selective sweeps found in invasive triploids were actual targets of selection; more likely these signals would derive from the process of establishment of triploids from diploid ancestors. It is nevertheless interesting that one enriched GO term found in triploids is ‘development of secondary sexual characteristics’. A previous study has shown that the ratio of males among invasive triploid C. auratusin Croatia was only 2.3% which was lower than that reported in the native range (Gui & Zhou, 2010; Jakovlic & Gui, 2011). It would be of interest to further investigate the ratio of males in Tibet.