Fragmentation can increase spatial genetic structure without decreasing pollen-mediated gene flow in a wind-pollinated tree

Abstract

Fragmentation reduces population sizes, increases isolation between habitats and can result in restricted dispersal of pollen and seeds. Given that diploid seed dispersal contributes more to shaping fine-scale spatial genetic structure (SGS) than haploid pollen flow, we tested whether fine-scale SGS can be sensitive to fragmentation even if extensive pollen dispersal is maintained. Castanopsis sclerophylla (Lindley & Paxton) Schottky (Fagaceae), a wind-pollinated and gravity seed-dispersed tree, was studied in an area of southeast China where its populations have been fragmented to varying extents by human activity. Using different age classes of trees in areas subject to varying extents of fragmentation, we found no significant difference in genetic diversity between prefragmentation vs. postfragmentation C. sclerophylla subpopulations. Genetic differentiation among postfragmentation subpopulations was also only slightly lower than among prefragmentation subpopulations. In the most fragmented habitat, selfing rates were significantly higher than zero in prefragmentation, but not postfragmentation, cohorts. These results suggest that fragmentation had not decreased gene flow among these populations and that pollen flow remains extensive. However, significantly greater fine-scale SGS was found in postfragmentation subpopulations in the most fragmented habitat, but not in less fragmented habitats. This alteration in SGS reflected more restricted seed dispersal, induced by changes in the physical environments and the prevention of secondary seed dispersal by rodents. An increase in SGS can therefore result from more restricted seed dispersal, even in the face of extensive pollen flow, making it a sensitive indicator of the negative consequences of population fragmentation.

Population at the edge: increased divergence but not inbreeding towards northern range limit in Acer campestre

“…Nonetheless, because empirical studies provided a mixed support for this prediction, it was argued that in many cases populations experienced fragmentation only recently, so that the observed inbreeding level still reflects more the historical conditions rather than the reduced population size (Aguilar et al 2008). Alternatively, fragmentation did not lead to increased inbreeding because of longdistance gene flow (Wang et al 2011;Leonardi et al 2012). In our study, however, A. campestre represents a stable distribution, with northern populations being naturally fragmented long enough to assure inbreeding to accumulate.…”

Constraints to and conservation implications for climate change adaptation in plants

“…However there is evidence that gene flow dynamics may also be relatively robust to habitat fragmentation (e.g. Lowe et al 2005a;Wang et al 2011;Breed et al 2012b). In a study into the effects of habitat fragmentation on genetic structure of the wind-pollinated, gravity seed-dispersed chinquapin tree (Castanopsis sclerophylla) no significant difference was observed in genetic diversity or inbreeding between pre-and post-fragmented populations, and this was attributed to extensive pollen dispersal .…”

Ecological succession of a natural community of Castanopsis sclerophylla on Laoshan Island, China

“…On Laoshan Island, both stock and seedling plants of C. sclerophylla have genetic diversity of over 88.9 %, making 81 % of its genetic variation from within the population (Shi 2008). Population fragmentation significantly reduces population genetic structure, but as a whole, gene flow does not change (Wang et al 2011(Wang et al , 2012. For C. sclerophylla, pollen spread has become more efficient although seed transmission has been restricted concomitant with the fragmentation, which has compensated for the disadvantage from restricted seed transmission.…”

“…For C. sclerophylla, pollen spread has become more efficient although seed transmission has been restricted concomitant with the fragmentation, which has compensated for the disadvantage from restricted seed transmission. The research of Shi (2008) and Wang (2012Wang ( , 2011, however, encompassed the genetic structure of C. sclerophylla on the larger scale of the whole Qiandao Lake region. On Laoshan Island, however, we believe the frequent genetic exchange within the community has prevented genetic drift, which has brought about low genetic variation within a short period, but the long-term inbreeding will increase the risk of within-community genetic drift and retrogression of the genetic structure.…”