Mary Jo W. Godt

Factors influencing levels of genetic diversity in woody plant species

The plant allozyme literature was reviewed to: (1) compare genetic diversity in long-lived woody species with species representing other life forms, and (2) to investigate whether the levels and distribution of genetic diversity in woody species are related to life history and ecological characteristics. Data from 322 woody taxa were used to measure genetic diversity within species, and within and among populations of species. Woody species maintain more variation within species and within populations than species with other life forms but have less variation among populations. Woody species with large geographic ranges, outcrossing breeding systems, and wind or animal-ingested seed dispersal have more genetic diversity within species and populations but less variation among populations than woody species with other combinations of traits. Although life history and ecological traits explain a significant proportion (34%) of the variation among species for the genetic parameters measured, a large proportion of the interspecific variation is unexplained. The specific evolutionary history of each species must play an important role in determining the level and distribution of genetic diversity.

Allozyme diversity in the endangered pitcher plant Sarracenia rubra ssp. Alabamensis (Sarraceniaceae) and its close relative S. rubra ssp. rubra.

Genetic variability in the federally endangered pitcher plant Sarracenia rubra ssp. alabamensis was assessed in eight Alabama populations using starch gel electrophoresis. Ten populations of the more widespread Sarracenia rubra ssp. rubra were sampled in the southeastern United States for comparison. Fifteen allozyme loci representing 13 enzyme systems were scored for each species. In contrast to S. oreophila and S. jonesii, two previously analyzed endangered pitcher plants, genetic diversity was high for both S. rubra subspecies. Within ssp. alabamensis the percentage polymorphic loci (Ps) was 80.0, the mean number of alleles per polymorphic locus was (APs) = 2.58, and expected heterozygosity (Hes) was 0.209. Genetic diversity was slightly lower for ssp. rubra (Ps = 73.3, APs = 2.91, and Hes = 0.177). The proportion of total genetic diversity found among populations was fairly low for both species (GST = 0.09 for ssp. alabamensis and 0.14 for ssp. rubra). Little genetic divergence has occurred between the two subspecies as indicated by the lack of diagnostic alleles, the proportion of total genetic diversity between taxa (GST = 0.09), and the genetic identity estimate (I = 0.90). The relatively high genetic diversity found for ssp. alabamensis indicates that the maintenance of its evolutionary potential is possible if population sizes are maintained or increased. Low levels of genetic diversity found within small Georgia ssp. rubra populations indicate that genetic erosion may increase extinction risks for these populations.

A ROLE FOR NONADAPTIVE PROCESSES IN PLANT GENOME SIZE EVOLUTION

Genome sizes vary widely among species, but comprehensive explanations for the emergence of this variation have not been validated. Lynch and Conery (2003) hypothesized that genome expansion is maladaptive, and that lineages with small effective population size (Ne) evolve larger genomes than those with large Ne as a consequence of the lowered efficacy of natural selection in small populations. In addition, mating systems likely affect genome size evolution via effects on both Ne and the spread of transposable elements (TEs). We present a comparative analysis of the effects of Ne and mating system on genome size evolution in seed plants. The dataset includes 205 species with monoploid genome size estimates (corrected for recent polyploidy) ranging from 2Cx = 0.3 to 65.9 pg. The raw data exhibited a strong positive relationship between outcrossing and genome size, a negative relationship between Ne and genome size, but no detectable Ne× outcrossing interaction. In contrast, phylogenetically independent contrast analyses found only a weak relationship between outcrossing and genome size and no relationship between Ne and genome size. Thus, seed plants do not support the Lynch and Conery mechanism of genome size evolution. Further work is needed to disentangle contrasting effects of mating systems on the efficacy of selection and TE transmission.

Patterns and levels of pollen mediated gene flow in Lathyrus latifolius

While gene flow can be an important force in evolution, few direct measures are available in the plant literature. Descriptions of gene movement within populations are more common, but have primarily involved crop species and artificially constructed populations. In this study, fractional paternity procedures were used to examine patterns of pollen movement over two years within two sites of Lathyrus latifolius, a bumblebee‐pollinated, self‐compatible perennial legume. Study sites consisted of 15 to 23 semi‐discrete flowering patches that contained 1 to 29 distinct genotypes. Distributions of gene movement distances within the study sites differed significantly from that expected under random mating. Mean gene movement was 14 m. On average, 17.6% (range = 0‐52%) of matings occurred within a flowering patch. Outcrossing rates, estimated from paternity analysis, ranged from 0.87 to 0.90 across sites and years. Significant heterogeneity occurred among maternal individuals with respect to outcrossing and immigration rates, indicating that mating patterns were idiosyncratic to each plant. Apparent rates of pollen flow into the sites ranged from 5 to 15%, while estimates of total pollen flow into sites ranged from 16 to 46%. Significant increases in immigration rates between years were associated with decreases in the density of flowering plants.

Allozyme diversity in the federally threatened golden paintbrush, Castilleja levisecta (Scrophulariaceae)

Castilleja levisecta (Scrophulariaceae), the golden paintbrush, is an insect-pollinated herbaceaous perennial found in the Pacific Northwest. Currently restricted to two island populations off British Columbia and nine populations (eight on islands) in Washington, C. levisecta is a rare species threatened with extinction. Allozymes were used to describe genetic diversity and structure in these eleven populations. Despite its threatened status and small geographic range, exceptionally high levels of genetic diversity are maintained within C. levisecta. All sixteen of the loci resolved were polymorphic within the species (Ps=100%), while the mean percentage of loci polymorphic within populations (Pp) was 65.7%. The mean number of alleles per polymorphic locus (APs) was 2.94 within the species and averaged 2.38 within populations (APp). Genetic diversity (Hes) was 0.285 for the species, whereas mean population genetic diversity (Hep) was 0.213. Smaller populations had, on average, fewer observed alleles and less genetic diversity. A significant negative correlation (r = −0.72) was found between genetic identity and geographic distance, indicating reduced gene flow between distant populations. The most geographically isolated population was one of the larger populations, one of the most genetically diverse and the most genetically divergent. A wide range of pairwise population genetic identities (I = 0.771 − 0.992) was found, indicating considerable genetic divergence between some populations. Overall, 19% of the total genetic diversity was distributed among populations. Results of this survey indicate that genetic augmentation of existing populations is unnecessary. The high allelic diversity found for the species and within its populations holds promise for conservation and restoration efforts to save this rare and threatened plant species.

Genetic diversity and morphological differentiation in Liatris helleri (Asteraceae), a threatened plant species

Liatris helleri (Asteraceae) is an insect-pollinated herbaceous perennial endemic to several high-elevation sites in the Blue Ridge Mountains of North Carolina. Allozymes were used to describe the genetic diversity and population structure in nine populations of this rare, federally listed species. Differences in leaf morphology were also examined for greenhouse-grown plants representing several populations. The proportion of the total genetic diversity found among populations, as indicated by the allozyme data, was 16%. Higher levels of population differentiation were found for differences in leaf shape; population of origin accounted for 37% of the variation in maximum leaf width, while families within populations accounted for 7%. In contrast to many endemic species, L. helleri maintains fairly high levels of genetic diversity. For the species, the percent polymorphic loci was 87.5, the average number of alleles at variable loci was 3.00 and the gene diversity was 0.276. Mean population values were percent polymorphic loci =58.4, mean number of alleles per polymorphic locus =2.59 and gene diversity =0.219. The estimated gene flow was low (Nm=0.85–1.32) and a relatively high correlation (r=0.55; p<0.005) was found between linear geographic and genetic distance. This suggests that the populations are partially isolated by distance, despite the limited range (<60 km) of the species. We recommend that population distinetiveness be maintained in restoration efforts.

Conservation of genetic diversity in old-growth forest communities of the southeastern United States

Abstract Question: How do studies of the distribution of genetic diversity of species with different life forms contribute to the development of conservation strategies? Location: Old-growth forests of the southeastern United States. Methods: Reviews of the plant allozyme literature are used to identify differences in genetic diversity and structure among species with different life forms, distributions and breeding systems. The general results are illustrated by case studies of four plant species characteristic of two widespread old-growth forest communities of the southeastern United States: the Pinus palustris - Aristida stricta (Longleaf pine - wiregrass) savanna of the Coastal Plain and the Quercus - Carya - Pinus (Oak-hickory-pine) forest of the Piedmont. Genetic variation patterns of single-gene and quantitative traits are also reviewed. Results: Dominant forest trees, represented by Pinus palustris (longleaf pine) and Quercus rubra (Northern red oak), maintain most of their genetic diversity within their populations whereas a higher proportion of the genetic diversity of herbaceous understorey species such as Sarracenia leucophylla and Trillium reliquum is distributed among their populations. The herbaceous species also tend to have more population-to-population variation in genetic diversity. Higher genetic differentiation among populations is seen for quantitative traits than for allozyme traits, indicating that interpopulation variation in quantitative traits is influenced by natural selection. Conclusion: Developing effective conservation strategies for one or a few species may not prove adequate for species with other combinations of traits. Given suitable empirical studies, it should be possible to design efficient conservation programs that maintain natural levels of genetic diversity within species of conservation interest.

Low levels of allozyme differentiation betweenPyxidanthera (pyxie-moss) taxa (Diapensiaceae)

TwoPyxidanthera morphs of questionable taxonomic rank are described in the eastern United States. We analyzed leaf samples from ninePyxidanthera populations (four of each morph and one with intermediate morphology) for 13 allozyme loci. Our results do not support differentiation of the two morphs at the species level. Mean genetic identity among populations was high (Ī = 0.97), and typical of that found for conspecific populations. The proportion of total genetic diversity found among populations was low (GST = 0.079). Several low frequency alleles were confined to each of the morphs, being found in some of the populations of each morph, but not all.