4.2 Genetic changes in invasive diploid populations
The loss of genetic diversity within invasive populations resulting from genetic bottlenecks can be prevented or even reversed by multiple introductions from the same or distinct sources, which have been reported in many animals and plants, such as Pseudorasbora parva ,Coptotermes formosanus , Aedes albopictus , Ambrosia artemisiifolia and Rosa rugosa (Baltazar-Soares et al., 2020; Blumenfeld et al., 2021; Kelager et al., 2013; Sherpa et al., 2019; van Boheemen et al., 2017). The invasive C. auratus in Tibet was introduced from different sources, which was also evidenced by phylogenetic analysis of diploid mitogenomes in this study, showing that invasive individuals were clustered into two regionally defined lineages of native individuals (i.e., the mid Yangzte River, the lower Yangzte River and Huai River). The two lineages showed a low but significant genetic differentiation (F ST: 0.0449) estimated based on genomic SNPs. In general, recent hybrid populations from diverse intraspecific source populations should possess higher genetic diversity than that of individual source populations (Baltazar-Soares et al., 2020; Simon et al., 2011; Smith et al., 2020). However, the genetic diversity of invasive diploid population of C. auratus was only slightly (9.0%) higher than that of source populations. Interestingly, at the individual level, the genome-wide observed heterozygosity of invasive individuals was significantly (21.4%) lower than that of source individuals (Figure 5a). The discordance between genetic diversity at the population and individual levels suggests that the total invasive population derives from genetically distinct sources, which may mask loss of variation at the population level during invasion. Nevertheless, the observed low levels of inbreeding suggest that founder events have still not had a significant biological impact. In that sense, the results provide an illustration of factors that may cause biological invasions to be associated with lower loss of genetic variation than expected (Blumenfeld et al., 2021; Facon et al., 2006). On a longer time scale, triploid C. auratus now dominate the market (Liu, 2010), and the LL wetland is padlocked since becoming a natural reserve zone since 1999 (Chen et al. 2018). Therefore, the diploid invasive population is expected to receive very low supplements of new invaders recently and in the future, which could accelerate loss of genetic diversity.