Genetic diversity in Leavenworthia populations with different inbreeding levels

Abstract

Levels of neutral genetic diversity within and between populations were compared between outcrossing (self-incompatible) and inbreeding populations in the annual plant genus Leavenworthia. Two taxonomically independent comparisons are possible, since self-incompatibility has been lost twice in the group of species studied. Within inbred populations of L. uni£ora and L. crassa, no DNA sequence variants were seen among the alleles sampled, but high diversity was seen in alleles from populations of the outcrosser L. stylosa, and in self-incompatible L. crassa populations. Diversity between populations was seen in all species. Although total diversity values were lower in the sets of inbreeding populations, between-population values were as high, or higher, than those in the outcrossing taxa. Possible reasons for these diversity patterns are discussed. As the e¡ect of inbreeding appears to be a greater than twofold reduction in diversity, we argue that some process such as selection for advantageous mutations, or against deleterious mutations, or bottlenecks occurring predominantly in the inbreeders, appears necessary to account for the ¢ndings. If selection for advantageous mutations is responsible, it appears that it must be some form of local adaptive selection, rather than substitution of alleles that are advantageous throughout the species. This is consistent with the ¢nding of high between-population diversity in the inbreeding taxa.

A Metapopulation Perspective on Genetic Diversity and Differentiation in Partially Self-Fertilizing Plants

“…First, data on allozyme variation in both plants (Hamrick andGodt 1990, 1996) and animals (Jarne 1995) have shown that inbreeding species have lower neutral genetic diversity within populations than more outcrossing taxa. Recent studies, using molecular data have verified this observation and also shown that neutral genetic diversity at the species level is often lower in inbreeding species, compared to closely related, more outcrossing species, although this effect is far less pronounced than within-population diversity (Bergelson et al 1998;Dvořák et al 1998;Liu et al 1998Liu et al , 1999Stephan and Langley 1998;Charlesworth and Pannell 2001). The second observation of the effects of mating system is that levels of genetic differentiation among populations, measured as F ST or related statistics, are generally higher in inbreeding species than in outcrossing species (Hamrick andGodt 1990, 1996;Charlesworth and Pannell 2001).…”

“…Recent work has extended these results to structured populations and shown that partial selfing can be incorporated into the standard population genetic theory for random mating populations if effective population size is reduced according to equation (1) (Maruyama and Tachida 1992;Tachida and Yoshimaru 1996). Several recent empirical studies, however, have found that reductions in genetic diversity within populations of inbreeding species are far greater than the expected reductions based on the reduced effective population size (Jarne 1995,Dvořák et al 1998Liu et al 1998Liu et al , 1999. At least three different processes are potentially able to cause these patterns.…”

“…A third possible explanation for the low observed levels of genetic diversity in inbreeding plants is variation in ecological or demographic processes such as migration or colonization (Fenster and Ritland 1992;Charlesworth et al 1997;Liu et al 1998;Charlesworth and Pannell 2001). For instance, selfing species often have reduced allocation to male function, such as pollen production and floral display in plants (Charlesworth and Pannell 2001).…”

“…Population bottlenecks are also expected, on theoretical grounds, to be more extreme in inbreeding plant species where, in principle, a single seed can establish a new population (Baker 1955;Schoen and Brown 1991). Thus, it is possible that metapopulation dynamics (i.e., recurrent extinctions and recolonizations) can explain the observed reductions in genetic diversity within local populations of selfing species (Charlesworth et al 1997;Liu et al 1998;Pannell and Charlesworth 2000).…”

Cytonuclear dynamics in selfing populations under selection

“…Although predominantly selfing species often have less genetic variation than cross-fertilizing species in nuclear genome (Baudry et al, 2001;Graustein et al, 2002;Hamrick and Godt, 1996;Liu et al, 1999Liu et al, , 1998Savolainen et al, 2000; but see Wright et al, 2002), substantial genetic variations do exist in highly selfing populations (Hedrick, 1998). The maintenance of nuclear polymorphisms has been the subject of a few studies and the exact conditions have been obtained Hedrick, 1998).…”