Landscape connectivity of Cercidiphyllum japonicum, an endangered species and its implications for conservation
Introduction
The use of ecological information and historical climatic and environmental data to guide the construction of appropriate phylogenetic and demographic models has added to our understanding of the role of particular geological barriers and climatic changes in intraspecific divergence (Chan et al., 2011). Recent approaches have been building upon the use of species distribution models (SDMs) by coupling them with coalescent analyses based on genetic data that allow for a better understanding of how demographic processes impact (or impacted) populations, both now and in the past (Knowles and Alvarado-Serrano, 2010, Brown and Knowles, 2012, Gehara et al., 2013, He et al., 2013, Dellicour et al., 2014). With the advent of the use of habitat heterogeneity as a friction layer with geospatial and environmental data, several approaches such as the least cost path (LCP) method and circuit theory developed using GIS-based methods or data have played an important role in many fields of biogeography (Vignieri, 2005, McRae and Beier, 2007, Graves et al., 2014). Notably, the dispersal network tool can be used to visualize dispersal corridors across landscapes and identify biogeographic barriers (Chan et al., 2011, Zeller et al., 2012, Brown, 2014). The GIS-based method can be useful and effective in revealing important information relating to evolution and conservation, especially in regard to endangered species (Brown and Yoder, 2015, Huerta-Ramos et al., 2015).
The cool-temperate deciduous forests in mainland China were generally believed to have retreated southward at the LGM. Since the Holocene, they expanded their range northward due to warmer and wetter conditions into the previously uninhabitable northern region, reaching their maximum geographical range around the mid-Holocene (Fang, 1991, Wang and Sun, 1994, Yu et al., 2000, Harrison et al., 2001, Ni et al., 2006). Recently, growing phylogeography studies have localized recurring glacial refugia in this region, such as at the Hengduan Mountains, the Qinling Mountains, and the Daba Mountains (Zhang et al., 2006, Zhang et al., 2006, Li et al., 2011, Qiu et al., 2011). However, little is still known about the recolonization route of the cool-temperate deciduous forests during the late Quaternary. The main objective of this paper is to localize their dispersal corridors during the late Quaternary using a model organism.
Cercidiphyllum japonicum is a tall canopy tree whose range extends widely eastward from southwest of the Sichuan Basin to the coastline of eastern China at a latitude between 25°N and 36°N; it is usually found in small discrete populations (Isagi et al., 2005, Krassilov, 2010). The species is well-known for its shortage of seedlings in its habitat resulting from relatively low seed fertility. In recent years, due to over-exploitation and human disturbance, the species' populations have diminished sharply and become further fragmented, and it is now on the edge of extinction in China (Wan and Zhang, 2002). The preliminary SDMs data in conjunction with molecular evidence from previous work on the species strongly suggest its recolonization of areas north of the Yangtze after the last glacial — that is, following the retreat of the arid steppe and desert vegetation there (Winkler and Wang, 1993, Yu et al., 2000, Harrison et al., 2001). This is the principal reason why we select C. japonicum as a model to identify their dispersal corridors. Moreover, previous work on the maternal chloroplast (cp) and biparental nuclear (n) DNA of the species emphasized various inheritance and dispersal features. Using landscape genetics approaches, we expected to be able to infer more comprehensive and convincing dispersal corridors. In addition, fossil evidence identified Cercidiphyllum as a dominant component of Tertiary forest and woody pioneer communities throughout the Northern Hemisphere (Mai, 1995, Meyer and Manchester, 1997, Manchester et al., 2009). Substantial fossil records would provide actual evidence for the dispersal hypothesis.
Here, we aimed to estimate the distributional changes of C. japonicum and identify its dispersal corridors during the late Quaternary. Furthermore, we expected to reveal some valuable implications for the evolutionary history and conservation priority of the endangered plant.
Section snippets
Determination of the shared haplotypes
When the integrated LCP method is used to localize the movement pathway based on molecular data, one premise is that populations with shared haplotypes usually experienced dispersal between sample localities (Chan et al., 2011, Yu et al., 2015). To explore the possible dispersal routes of C. japonicum, the populations that have shared haplotypes needed to be determined first. In a previous phylogeographical study, Qi et al. (2012) collected samples from 33 populations from mainland China and
The distribution change since the LGM
Due to the cycles of glaciation in the Quaternary period, species' distribution was commonly altered within the temperate regions, as observed in this study. When comparing the distribution of the species in relation to the three time periods, we identified differences in the range of C. japonicum: (1) from the LGM to the present, the range showed a marked tendency of expansion with the distribution area increasing by about 30%, though suitable habitat contracted in the northeast distribution
Discussion
In consideration of the great differences in genetic connectivity based on the cpDNA and nDNA haplotypes, we are more inclined to estimate dispersal routes based on the inheritance of the maternal genome than the biparental nDNA. In C. japonicum, cpDNA is inherited maternally through seed while nDNA is paternally inherited and dispersed through pollen and seed (Qi et al., 2012). Because seed dispersal distance is generally greatly reduced relative to that of pollen (Ennos, 1994), it is expected
Acknowledgments
This research was supported by the National Natural Science Foundation of China (grant no. 31360045).
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