Small effect of fragmentation on the genetic diversity of Dalbergia monticola, an endangered tree species of the eastern forest of Madagascar, detected by chloroplast and nuclear microsatellites

Abstract

Background and aims: The oriental forest ecosystem in Madagascar has been seriously impacted by fragmentation. The pattern of genetic diversity was analysed on a tree species, Dalbergia monticola, which plays an important economic role in Madagascar and is one of the many endangered tree species in the eastern forest.

Methods: Leaves from 546 individuals belonging to 18 small populations affected by different levels of fragmentation were genotyped using eight nuclear (nuc) and three chloroplast (cp) microsatellite markers.

Key results: For nuclear microsatellites, allelic richness (R) and heterozygosity (H(e,nuc)) differed between types of forest: R = 7.36 and R = 9.55, H(e,nuc) = 0.64 and H(e,nuc) = 0.80 in fragmented and non-fragmented forest, respectively, but the differences were not significant. Only the mean number of alleles (N(a,nuc)) and the fixation index F(IS) differed significantly: N(a,nuc) = 9.41 and N(a,nuc) = 13.18, F(IS) = 0.06 and F(IS) = 0.15 in fragmented and non-fragmented forests, respectively. For chloroplast microsatellites, estimated genetic diversity was higher in non-fragmented forest, but the difference was not significant. No recent bottleneck effect was detected for either population. Overall differentiation was low for nuclear microsatellites (F(ST,nuc) = 0.08) and moderate for chloroplast microsatellites (F(ST,cp) = 0.49). A clear relationship was observed between genetic and geographic distance (r = 0.42 P < 0.01 and r = 0.42 P = 0.03 for nuclear and chloroplast microsatellites, respectively), suggesting a pattern of isolation by distance. Analysis of population structure using the neighbor-joining method or Bayesian models separated southern populations from central and northern populations with nuclear microsatellites, and grouped the population according to regions with chloroplast microsatellites, but did not separate the fragmented populations.

Conclusions: Residual diversity and genetic structure of populations of D. monticola in Madagascar suggest a limited impact of fragmentation on molecular genetic parameters.

Fig. 1

Map of Madagascar showing remaining primary vegetation (grey), location of the provenances (in bold) and populations (in italics) sampled throughout the eastern forest.

Fig. 2

Unrooted neighbor-joining tree drawn with Darwin 5.0.84 software (Perrier et al., 2003), with the matrix of genetic distances calculated using the Euclidian distance. Numbers at the base of the branches are percentages corresponding to the bootstrap values after 1000 replications (only bootstraps higher than 50 % are shown). (A) Nuclear microsatellites; (B) chloroplast microsatellites. Region abbreviations: C, Centre; N, North; CN, Centre-North; S, South. For landscape codes (1, 2, 3) see Table 1.

Fig. 3

Relationship between genetic and geographical distances among populations of Dalbergia monticola. Matrices of genetic distances were calculated using pairwise F_ST. (A) Nuclear microsatellites; (B) chloroplast microsatellites.