Genetic structure and outcrossing rates in Flourensia cernua (Asteraceae) growing at different densities in the South-western Chihuahuan Desert

Abstract

Backgrounds and aims: Flourensia cernua is a partially self-incompatible, wind-pollinated shrub that grows in two scrub types of contrasting densities. It was anticipated that differences in plant density would affect the amount of genotype availability, and thus higher outcrossing rates and less genetic differentiation would be found at high-density sites.

Methods: At five high-density sites and at five low-density sites, 11 allozyme loci were analysed in adults. Outcrossing rates were estimated using five allozyme loci sampled from eight families from each scrub type.

Key results: High levels of genetic variation were found at all sites (ranging from P = 82-100 %, He = 0.33-0.45, and Ho = 0.4-0.59). Heterozygotes were found in excess (FIS = -0.15 +/- 0.06 s.d.), suggesting that natural selection favours heterozygosity, and there was little differentiation between sites (FST = 0.08 +/- 0.02 s.d.). Life history attributes, such as long-lived habit and wide geographic distribution, as well as the presence of a self-incompatibility system may explain these results. Outcrossing rates did not differ from 1.0 in both scrub types, and there was no genetic differentiation between scrub types (FST = -0.01 +/- 0.004 s.d.).

Conclusions: The high rate of outcrossing favoured by partial incompatibility may generate unrestricted gene flow between scrub types and thus may explain the lack of differentiation between them. High heterozygosity could be expected in long-lived plants of arid zones as they confront a variable and stressing environment.

Fig. 1.

Aerial photograph of distribution of Flourensia cernua sites. High-density scrubs (A, B, C, D and E ) and low-density scrubs (F, G, H, I and J) in the southwestern Chihuahuan Desert (original scale 1:75 000, INEGI, 1986).

Fig. 2.

Cluster phenogram of Flourensia cernua populations in the southwestern Chihuahuan Desert. High-density scrubs (A, B, C, D and E) and low-density scrubs (F, G, H, I and J). The dendrogram was constructed using Nei's genetic distances (Nei, 1978) and UPGMA algorithms (Sokal and Micherner, 1958). UPGMA Cluster using Nei's (1978) original distance for each node is presented in italics; percentage of similar replicates (results from bootstrapping, 1000 permutations) are in bold.