Maintenance of species boundaries amid hybridization in two island gingers with similar ecological niches.

Understanding species boundaries maintenance in the face of hybridization/introgression is an intriguing yet complex topic in evolutionary biology. The underlying mechanisms, however, remain elusive. To address this, we propose to investigate the role of climatic shifts in shaping genetic structure and influencing species boundaries. We combine multilocus genetic data and species distribution modeling to explore how past and current climatic shifts affect the genetic structure and demographic history of two Taiwan endemic gingers, Zingiber pleiostachyum and Z. shuanglongense. We identified a well-delimited genetic structure with evidence of admixture, indicating incomplete reproductive isolation between the two gingers. This is likely due to secondary contact and range overlap during the last glacial maximum, leading to sporadic instances of hybridization. Niche overlap tests based on climate and soil data indicate that these two gingers occupy similar but nonidentical ecological niches. Furthermore, we found that the considerable differences in their current geographic distribution and altitude preferences might have resulted from different seed dispersal capabilities and competitive exclusion due to their similar niche preferences. Our results reveal a model where altitudinal differentiation and dispersal strategy synergistically reinforce the species divergence, thereby illuminating the importance of these factors in shaping and maintaining the island's biodiversity.

Local adaptation and migratory habits balance spatial-genetic structure between continental and insular chestnut tiger butterflies in East Asia.

Geographic and climatic differences between islands and continents may affect the evolution of their biota, and promote divergent selection in species distributed in both landscapes. To assess spatial-genetic structure, we genotyped 18 expressed sequence tag-simple sequence repeat (EST-SSR) loci and sequenced two mtDNA markers (ND5 and COI) and one nuclear marker (EF1α) in two subspecies of the butterfly Parantica sita. Compared with nuclear markers, mtDNA had a stronger signal of population structure. Approximate Bayesian computation (ABC) suggested that a continuous-gene-flow model best described the data. According to this model, the two subspecies diverged approximately 23.1 kya, with 10 times more introgression from the continental (ssp. sita) to the insular subspecies (ssp. niphonica) than vice versa. Ecological niche modelling was performed to predict the paleo- and current potential distributions and elucidate the geohistorical process, which revealed a northeastern, insular origin. Winter precipitation and annual temperature range were the main determinants of the subspecies distributions. Maximum-likelihood population-effects models showed that the population differentiation of the insular and continental subspecies was primarily affected by environmental resistance and local climate. Sex-biased migration capacity and long-term precipitation-driven divergence between the continental and insular lineages shaped the current genetic structure of P. sita. Evidence from the nuclear markers confirmed intersubspecific gene flow despite adaptive divergence between the subspecies. These results imply that the continental subspecies is still capable of returning to the island and introgressing with the insular subspecies.

Environmental Heterogeneity Leads to Spatial Differences in Genetic Diversity and Demographic Structure of Acer caudatifolium.

Under climate fluctuation, species dispersal may be disturbed by terrain and local climate, resulting in uneven spatial-genetic structure. In addition, organisms at different latitudes may be differentially susceptible to climate change. Here, we tracked the seed dispersal of Acer caudatifolium using chloroplast DNA to explore the relationships of terrain and local climate heterogeneity with range shifts and demography in Taiwan. Our results showed that the extant populations have shifted upward and northward to the mountains since the Last Glacial Maximum. The distributional upshift of A. caudatifolium is in contrast to the downward expansion of its closest relative in Taiwan, A. morrisonense. The northern populations of A. caudatifolium have acquired multiple-source chlorotypes and harbor high genetic diversity. However, effective gene flow between the north and south is interrupted by topography, geographic distance, north-south differences in October rainfall, and other climate heterogeneities, blocking southward genetic rescue. In addition, winter monsoon-driven rainfall may cause regional differences in the phenological schedule, resulting in adaptive effects on the timing of range shift and the genetic draft of chlorotype distribution. Terrain, distance, and local climate also differentiate the northernmost populations from the others, supporting the previous taxonomic treatment of Acer kawakamii var. taitonmontanum as an independent variety.

Differential genetic responses to the stress revealed the mutation-order adaptive divergence between two sympatric ginger species.

BACKGROUND: Divergent genetic responses to the same environmental pressures may lead sympatric ecological speciation possible. Such speciation process possibly explains rapid sympatric speciation of island species. Two island endemic ginger species Zingiber kawagoii and Z. shuanglongensis was suggested to be independently originated from inland ancestors, but their island endemism and similar morphologies and habitats lead another hypothesis of in situ ecological speciation. For understanding when and how these two species diverged, intraspecific variation was estimated from three chloroplast DNA fragments (cpDNA) and interspecific genome-wide SNPs and expression differences after saline treatment were examined by transcriptomic analyses. RESULTS: Extremely low intraspecific genetic variation was estimated by cpDNA sequences in both species: nucleotide diversity π = 0.00002 in Z. kawagoii and no nucleotide substitution but only indels found in Z. shuanglongensis. Nonsignificant inter-population genetic differentiation suggests homogenized genetic variation within species. Based on 53,683 SNPs from 13,842 polymorphic transcripts, in which 10,693 SNPs are fixed between species, Z. kawagoii and Z. shuanglongensis were estimated to be diverged since 218~ 238 thousand generations ago (complete divergence since 41.5~ 43.5 thousand generations ago). This time is more recent than the time of Taiwan Island formation. In addition, high proportion of differential expression genes (DEGs) is non-polymorphic or non-positively selected, suggesting key roles of plastic genetic divergence in broaden the selectability in incipient speciation. While some positive selected DEGs were mainly the biotic and abiotic stress-resistance genes, emphasizing the importance of adaptive divergence of stress-related genes in sympatric ecological speciation. Furthermore, the higher proportional expression of functional classes in Z. kawagoii than in Z. shuanglongensis explains the more widespread distribution of Z. kawagoii in Taiwan. CONCLUSIONS: Our results contradict the previous hypothesis of independent origination of these two island endemic ginger species from SE China and SW China. Adaptive divergent responses to the stress explain how these gingers maintain genetic differentiation in sympatry. However, the recent speciation and rapid expansion make extremely low intraspecific genetic variation in these two species. This study arise a more probable speciation hypothesis of sympatric speciation within an island via the mutation-order mechanism underlying the same environmental pressure.