Introductions
Hybridization may cause significant impacts on the types and levels of genetic divergence between species (Abbott et al. 2013; Payseur& Rieseberg 2016; Zhang et al. 2016). This divergence includes neutral divergence, adaptive introgression and coevolution, which may accumulate in different ways. Some species may acquire novel characteristics or phenotypes through interactions with hybrids, including advantages derived from genetic recombination and disadvantages resulting from allelic or environmental incompatibilities (Abbott et al. 2013; Meier et al. 2017). In addition to its genetic consequences, hybridization can influence the species composition of communities. When two species with different populations exist in the same habitat, individuals from smaller populations may mate more frequently with members of larger populations, resulting in the dilution of the smaller populations. Some endangered species may become extinct as a result of hybridization (Levin et al. 1996).
For the past several decades, hybridization was considered to be relatively rare, especially in animals (Abbott et al. 2013; Heliconius Genome 2012). However, recent studies have suggested that hybridization in animals is more common than previously thought, providing primary material for both speciation and evolution (Abbott et al. 2013; Kulmuni& Pamilo 2014; Song et al.2011). Approximately 10-30% of animal and plant species hybridize regularly. High rates of hybridization have been reported in many groups, including birds, insects and fishes (Abbott et al. 2013; Capblancq et al. 2015; Pennisi 2016). Incomplete barriers between species may lead to adaptive introgression through the transmission of beneficial alleles between species via backcrosses, which may occur between closely related species.
Two closely related termite species, Reticulitermes flavicepsOshima and Reticuliterme chinensis Snyder, are common forest and structural insect pests in China (Liet al. 2014). They are consistently recognized as distinct species on the basis of multiple criteria, including morphology, biology (colony life history) and mitochondrial genome levels (Bourguignon et al. 2016). In terms of morphological characteristics, the colour of the R. flavicepsalate pronotum is yellow, but that of R. chinensis is black. The eclosion and swarming of R. chinensis occur in the same year at intervals of approximately half a month, but in R. flaviceps , eclosion occurs in the first year, and swarming occurs in the second year, with intervals of approximately 4 months. Despite the differences in swarming times, some overlap of dispersal periods and the reproductive season persists in the two species. More importantly,R. flaviceps and R. chinensis share stable habitats, including nesting and foraging sites. Thus, reproductive individuals of the two termite species may encounter each other in nature while searching for mates and nest sites. In preliminary laboratory experiments, we found that a preference for conspecific partners was absent when these species encountered each other during the reproductive season. Pairs established in the laboratory can produce surviving hybrid larvae (Wu et al. 2020), suggesting that hybridization and adaptive introgression occur in the context of incomplete barriers between R. flavicepsand R. chinensis. However, whether hybridization and adaptive introgression occur in nature is still unclear.
Here, we combined microsatellite (simple sequence repeat SSR) genotyping with mitochondrial DNA sequencing to analyse the genetic structure of the sympatric species R. flaviceps and R. chinensis and to answer the following three questions: (1) whether hybridization and adaptive introgression occur between the two termite species in nature; (2) whether similar genetic differentiation exists in their mitochondrial DNA (mtDNA) and SSR (nDNA); and (3) the effect of hybridization and introgression on the genetic structure and evolution of the termites.
Materials and methods