Evidence for ancient genetic subdivision among recently fragmented populations of the endangered shrub Grevillea caleyi (Proteaceae)

Abstract

The genetic effects of population fragmentation cannot be interpreted without understanding the underlying pattern of genetic variation resulting from historic population processes. We used AFLP markers to determine genetic structure and distribution of genetic diversity among populations of an endangered Australian shrub Grevillea caleyi (Proteaceae). Populations that occurred historically on four ridges have new been fragmented to varying degrees, producing some large, relatively pristine populations and very small populations consisting of fewer than 10 adult plants. We found marked population genetic structure (65.9% of genetic variation was among populations) and a significant relationship between genetic and geographic distance (r m ¼ 0.564, P ¼ 0.004). However, only 14% of overall genetic differentiation was attributable to variation among ridges, compared with 52% among populations within ridges. Moreover, genetic diversity within samples of plants did not vary with either population size or degree of isolation. Thus, the present genetic structure of populations is probably almost entirely the product of historical events. Fine-scale structuring within populations prior to fragmentation may have been caused by limited seed and pollen dispersal, despite a complex suite of (mostly avian) pollinators, and a mixed mating system that allows a large amount of selfing. The combined effects of adult longevity and a soil-stored seed bank may have buffered the recently fragmented populations against the effects of dramatic reductions in numbers of adult plants.

Genetic variation and differentiation in populations of a threatened tree, Magnolia stellata: factors influencing the level of within-population genetic variation

“…Since it is more difficult to measure the Genetic variation in Magnolia stellata populations I Tamaki et al degree of population isolation than population size, good measures for quantifying the degree of population isolation have not yet been established. However, we detected significant relationships between the degree of population isolation and genetic variation, using the number of surrounding populations as the measure of isolation, whereas the distance from the nearest population was used in many previous studies (for example, Llorens et al, 2004;Galeuchet et al, 2005;Lu et al, 2005). Thus, the detection of a significant relationship in this study, and the failure of many of the cited studies to detect such relationship, supports the assertion by Moilanen and Nieminen (2002) that the number of surrounding populations is a much better indicator of isolation than the distance from the nearest population.…”

“…The simplest way of doing this is to measure the distance to the nearest population. This approach has been used by various authors (for example, Llorens et al, 2004;Galeuchet et al, 2005;Lu et al, 2005), but few have found significant relationships between this parameter and genetic variation (Lu et al, 2005), probably because it provides a too simplistic indication of isolation (Moilanen and Nieminen, 2002). The cited authors studied possible connectivity measures in spatial ecology and recommended use of a 'buffer measure' based on the number of populations within a given radius around the focal population rather than simply the distance to the nearest population.…”

Limiting inbreeding in disjunct and isolated populations of a woody shrub

“…Several studies of plant species with disjunct distributions have found that genetic diversity is unrelated to census population sizes (Hoebee and Young 2001; Llorens et al. 2004; Leimu and Mutikainen 2005; Mathiasen et al. 2007; Sampson et al.…”

“…For example, large seed banks can buffer against loss of diversity following population size reduction and inbreeding (Llorens et al. 2004); fitness can be maintained despite high selfing rates through purging of lethals and high seed production (Sampson et al. 2014); self‐incompatibility can promote diversity through multiple paternity in small, dense populations (Llaurens et al.…”

Does genetic variation and gene flow vary with rarity in obligate seeding Persoonia species (Proteaceae)?

“…One possible explanation is that our estimate of population size is likely to underestimate the effective population size of these Persoonia species, all of which can develop substantial seedbanks in the soil (Auld et al 2000). A persistent seed-bank provides the opportunity for gene flow over time (between the offspring of multiple cohorts) and can potentially buffer populations against the effects of genetic drift (Llorens et al 2004). Furthermore, there may not have been sufficient time to detect the recent effects of habitat fragmentation (<150 year ago) in these long-lived perennials (Rossetto et al 1995).…”