Genetic Introgression and Species Boundary of Two Geographically Overlapping Pine Species Revealed by Molecular Markers

Abstract

Gene introgression and hybrid barriers have long been a major focus of studies of geographically overlapping species. Two pine species, Pinus massoniana and P. hwangshanensis, are frequently observed growing adjacent to each other, where they overlap in a narrow hybrid zone. As a consequence, these species constitute an ideal system for studying genetic introgression and reproductive barriers between naturally hybridizing, adjacently distributed species. In this study, we sampled 270 pine trees along an elevation gradient in Anhui Province, China and analyzed these samples using EST-SSR markers. The molecular data revealed that direct gene flow between the two species was fairly low, and that the majority of gene introgression was intermediated by backcrossing. On the basis of empirical observation, the on-site distribution of pines was divided into a P. massoniana zone, a hybrid zone, and a P. hwangshanensis zone. STRUCTURE analysis revealed the existence of a distinct species boundary between the two pine species. The genetic boundary of the hybrid zone, on the other hand, was indistinct owing to intensive backcrossing with parental species. Compared with P. massoniana, P. hwangshanensis was found to backcross with the hybrids more intensively, consistent with the observation that morphological and anatomical characteristics of trees in the contact zone were biased towards P. hwangshanensis. The introgression ability of amplified alleles varied across species, with some being completely blocked from interspecific introgression. Our study has provided a living example to help explain the persistence of adjacently distributed species coexisting with their interfertile hybrids.

Importance of incomplete lineage sorting and introgression in the origin of shared genetic variation between two closely related pines with overlapping distributions

“…Together, the ABC and niche modeling therefore suggest that the species demographic history has been complex and probably involves multiple range contractions and expansions leading to successive periods of gene flow that were initiated before the LIG. The possibility of current and past secondary gene flow is also supported by recent studies showing, along elevation gradients, that the two species still hybridize at a low rate in intermediate altitude contact zones ( Luo and Zou, 2001 ; Zhang et al , 2014 ). This low introgression rate is more likely to be associated with postzygotic rather than prezygotic isolation.…”

EST‐SSR‐based landscape genetics ofPseudotaxus chienii, a tertiary relict conifer endemic to China

“…chienii at the species level. In comparison with other coniferous species (Table ), its average expected heterozygosity is lower than Torreya grandis ( He = 0.432) (Zeng et al., 2018), Pinus massoniana ( He = 0.5717) (Zhang et al., 2014), Picea abies ( He = 0.616) (Stojnić et al., 2019), and P . likiangensis ( He = 0.7186) (Cheng et al., 2014); close to P .…”

“…Pinus massoniana (He = 0.5717) (Zhang et al, 2014), Picea abies (He = 0.616) (Stojnić et al, 2019), and P. likiangensis (He = 0.7186) (Cheng et al, 2014); close to P. dabeshanensis (He = 0.36) (Zhang et al, 2016) and Amentotaxus argotaenia (He = 0.39) (Ruan et al, 2019); and higher than A. formosana (He = 0.1993) (Li et al, 2016), P. bungeana (He = 0.205) (Duan et al, 2017), and A. yunnanensis (He = 0.3343) (Li et al, 2016 history, natural selection, and mutation rate (Hamrick et al, 1992;Su et al, 2009;Wang et al, 2020).…”

Plant speciation across environmental gradients and the occurrence and nature of hybrid zones

“…Based on this type of genetic signature, it appears that hybrid zone movement across altitudinal gradients may have occurred for hybrid zones formed between Pinus massoniana and P. hwangshanensis in Anhui, China (Zhang et al, ), Populus angustifolia and P. fremontii in Utah, USA (Martinsen et al, ), Senecio aethnensis and S. chrysanthemifolius on Mount Etna, Sicily (James & Abbott, ; Chapman et al, ), and Senecio hercynicus and S. ovatus in Bavaria, Germany (Bog et al, ). In the case of Senecio , the genetic signatures of asymmetric introgression indicate that the introgressed lowland species, S. chrysanthemifolius and S. ovatus , are advancing to higher altitudes, possibly in response to recent climate warming, and displacing their higher altitude counterparts from these areas.…”