Variable rates of random genetic drift in protected populations of English yew: implications for gene pool conservation

Abstract

Protecting populations in their natural habitat allows for the maintenance of naturally evolved adaptations and ecological relationships. However, the conservation of genetic resources often requires complementary practices like gene banks, translocations or reintroductions. In order to minimize inbreeding depression and maximize the adaptive potential of future populations, populations chosen for ex situ conservation should be selected according to criteria that will result in a reduction of global coancestry in the population. Generally, large populations should reveal lower coancestry and higher genetic variation than small populations. If detailed knowledge about coancestry is lacking, census population number (N c ) can be used as a proxy for required characteristics. However, a simple measure of N c may be misleading in particular cases as genetic processes rely on effective population size (N e ) rather than N c and these two measures may differ substantially due to demographic processes. We used an example of English yew to address whether N c can be a good predictor of genetic parameters when used in conservation programs. Using microsatellite markers, we estimated allelic richness, inbreeding and coancestry coefficients of six relatively large yew populations in Poland. Each population was characterized by N e using the linkage disequilibrium method. Our results showed that populations of English yew were subject to substantial divergence and genetic drift, with both being inversely proportional to the effective subpopulation size (N e ). Additionally, allelic richness appeared proportional to N e but not to N c . However, the N e /N ratio differed greatly among populations, which was possibly due to different population histories. From the results we concluded that choosing source populations based only on their census size can be fairly misleading. Implications for conservation are briefly discussed.

Relative strength of fine-scale spatial genetic structure in paternally vs biparentally inherited DNA in a dioecious plant depends on both sex proportions and pollen-to-seed dispersal ratio

“…According to the sex allocation theory, females are generally less competitive than males and females of English yew may have a higher mortality (Iszkuło et al, 2009). As shown in our earlier study, both populations reveal a significant SGS at six nuclear microsatellite markers (Chybicki et al, 2011) as well as relatively high genetic variation and no signal of a recent bottleneck (Chybicki et al, 2012). In addition, Cisy Staropolskie showed low (that is, F = 0.04), but statistically significant inbreeding (Chybicki et al, 2011).…”

“…Our previous study showed, for example, that Cisy Staropolskie are characterised by apparently lower ratio of effective (N e ) to census population size (N) than Cisy w Czarnem (N e /N = 0.033 vs 0.40, respectively; Chybicki et al, 2012) that could not be explained simply by differences in sex proportions. In addition, population histories, including possible variation of sex ratios, remain unknown except for the genetic evidence of no recent bottleneck (Chybicki et al, 2012). Thus, although the observed SGS patterns seem to reflect differences in sex proportions between the populations, we postulate to treat the conclusions with caution.…”

“…To study SGS intensity in the mitochondrial vs nuclear genome, we used samples collected within two remnant populations of T. baccata well characterised for genetic structure in our previous studies (Chybicki et al, 2011(Chybicki et al, , 2012. Within each population, sex proportions were estimated based on direct observations of reproductive organs or remains of seed envelopes attached to branches.…”

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

“…Similar reasons could lead to the elevated inbreeding estimate in the case of Las Mariański, which represented the most admixed gene pool in the region of lower species abundance. Alternative explanation would be a specific colonisation history, if a few (related) adult trees gave a rise to the population (Pujol et al 2009;Chybicki et al 2012). However, because no signatures of a strong bottleneck were revealed, the hypothesis of bottleneck has no support from the data.…”

No reduction in genetic diversity of Swiss stone pine (Pinus cembra L.) in Tatra Mountains despite high fragmentation and small population size

“…Protection of populations in their natural habitat (in situ) maintains ecological and evolutionary processes, as well as adaptation (Chybicki et al 2012). However, the conservation of endangered and threatened tree species often requires proactive ex situ practices, such as the establishment of seed orchards, gene banks, or clone archives.…”