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