Population genetics

Although the study of plant population genetics may have no direct bearing on the economic development of Echinacea, it is a field directly applied to the analysis of genetic variation and to the conservation of both in situ and ex situ populations. Population genetic analyses elucidate patterns of genetic variation as it is distributed within and among populations of a species (Hamrick and Godt, 1996). These analyses rely on clear measures of genotypic and allelic diversity, traditionally estimated from the frequencies of co-dominant allozyme markers. Population structure, the departure from expected Hardy-Weinberg equilibrium within a population, is usually characterized by the well-known F-statistics developed by Wright (1951). (See Hartl and Clark (1997) for a detailed background.)

Allozyme data yielding estimates of genotypic frequency and allelic diversity have been collected for both of the federally endangered species, E. tennesseensis (Baskauf et al., 1994) and E. laevigata (Apsit and Dixon, 2001). Baskauf et al. (1994) compared population structure and diversity estimates of the endemic E. tennesseensis with that of its widespread congener, E. angustifolia. This comparative method was adopted in order to avoid the confounding effects of phylogenetic differences between species from different genera (Baskauf et al., 1994; Gitzendanner and Soltis, 2000). E. tennesseensis had significantly less genetic variability than did E. angustifolia (Baskauf et al., 1994). F-statistics indicated that less than 10% of the total genetic variation for each species was due to differences among populations (Baskauf et al., 1994). This is reasonably consistent with the findings of Kapteyn et al. (2002) and corresponds to the strong genetic similarity among populations for a given species per calculations of genetic identity (Baskauf et al., 1994).

Echinacea laevigata is the other rare and endangered taxon in the genus, with 24 known, extant populations endemic to the southeastern United States (Apsit and Dixon, 2001). Measures of both genotypic and allelic diversity indicated that the 11 populations sampled are well differentiated and that conservation of all populations would be the ideal approach to preserving the total range of genetic diversity of this species (Apsit and Dixon, 2001). Note that the "among population" component of variance in this study was 78%, indicating that most of the genetic variability was caused by differences between populations (Apsit and Dixon, 2001). This is dramatically different from the results of Baskauf et al. (1994) and Kapteyn et al. (2002) for other species. Many factors can contribute to population differentiation, including genetic drift resulting from reduced population size and limited gene flow from isolation. See Apsit and Dixon (2001) for an excellent discussion of such factors.

Wagenius (2000) studied the diversity and "fine-scale" population substructure (by mapping individual plants within subpopulations) of wild E. angustifolia in a fragmented prairie region of western Minnesota. He found a strong positive linear correlation of several important estimates of genetic diversity (proportion of polymorphism, allelic richness, and gene diversity) with population size, that is, the smaller the population, the less diversity (Wagenius, 2000). Small local populations also had more spatial genetic structure, indicating a reduction in random mating (Wagenius, 2000). Wagenius (2000) concluded that small population size was a factor in lowered fitness as measured by pollen limitation (the absence of compatible pollen inferred from style persistence) and progeny vigor. It would be interesting to see how values of genetic identity and interpopulation distances as measured by other molecular markers compared to these results.

A few comments on mating systems in Echinacea are warranted here. E. angustifolia has been shown to have the potential for a mixed mating system including xenogamy (out-crossing between individuals), geitonogamy (self-fertilization between florets of the same individual), and autogamy (self-fertilization within a floret) in a native prairie of southwestern South Dakota (Leuszler et al.,

1996). McGregor observed a small degree of self-mating in all Echinacea species (McGregor,

1997). The assumption of a self-incompatibility system (not yet genetically characterized) in Echinacea as in other Asteraceae is reasonable; however, given that incompatibility systems can be leaky, as demonstrated in other composites (Cheptou et al., 2002; Young et al., 2000) and by E. angustifolia, a mixed mating system in Echinacea would not be surprising. Also, reduced population size in out-crossing species can lead to increased self-crossing (Reinhartz and Les, 1994). To add to this complexity, E. laevigata reproduces clonally (Apsit and Dixon, 2001; Edwards, 1997), as do other species in cultivation (McKeown, unpublished observation). A detailed study of Echinacea mating systems will be essential to a full understanding of its native diversity.

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