Andrew Schnabel

Understanding the population genetic structure of Gleditsia triacanthos L.: seed dispersal and variation in female reproductive success

Fine‐scale genetic structuring is influenced by a variety of ecological factors and can directly affect the evolutionary dynamics of plant populations by influencing effective population size and patterns of viability selection. In many plant species, genetic structuring within populations may result from highly localized patterns of seed dispersal around maternal plants or by the correlated dispersal and recruitment of siblings from the same fruit. This fine‐scale genetic structuring may be enhanced if female parents vary significantly in their reproductive success. To test these hypotheses, we used genetic data from 17 allozyme loci and a maximum‐likelihood, ‘maternity‐analysis’ model to estimate individual female fertilities for maternal trees across a large number of naturally established seedlings and saplings in two populations of Gleditsia triacanthos L. (Leguminosae). Maximum‐likelihood fertility estimates showed that the three highest fertility females accounted for 58% of the 313 progeny at the first site and 46% of the 651 progeny at the second site, whereas 18 of 35 and 16 of 34 females, respectively, had fertility estimates that did not exceed 1%. Additional analyses of the second site found individual female fertility to vary significantly both within and among juvenile age classes. Female fertility at the first site was weakly correlated with maternal tree size and spatial location relative to the open, old‐field portions of the population, where the great majority of seedlings and saplings were growing, but no such correlations were found at the second site. Estimates of realized seed dispersal distances indicated that dispersal was highly localized at the first site, but was nearly random at the second site, possibly reflecting differences between the two sites in the behaviour of animal dispersers. The combined estimates of seed dispersal patterns and fertility variation are sufficient to explain previously described patterns of significant fine‐scale spatial genetic structure in these two populations. In general, our results demonstrate that effective seed dispersal distributions may vary significantly from population to population of a species due to the unpredictable behaviour of secondary dispersers. Consequently, the effects of seed dispersal on realized fine‐scale genetic structure may also be relatively unpredictable.

Cladistic biogeography of Gleditsia (Leguminosae) based on ndhF and rpl16 chloroplast gene sequences.

We used cladistic analysis of chloroplast gene sequences (ndhF and rpl16) to test biogeographic hypotheses in the woody genus Gleditsia. Previous morphological comparisons suggested the presence of two eastern Asian-eastern North American species pairs among the 13 known species, as well as other intra- and inter-continental disjunctions. Results from phylogenetic analyses, interpreted in light of the amount of sequence divergence observed, led to the following conclusions. First, there is a fundamental division of the genus into three clades, only one of which contains both Asian and North American species. Second, the widespread and polymorphic Asian species, G. japonica, is sister to the two North American species, G. triacanthos and G. aquatica, which themselves are closely related inter se, but are both polymorphic and paraphyletic. Third, the lone South American Gleditsia species, G. amorphoides, forms a clade with two eastern Asian species. Gleditsia thus appears to have only one Asian-North American disjunction and no intercontinental species pairs. Low sequence divergence between G. amorphoides and its closest Asian relatives implicates long-distance dispersal in the origin of this unusual disjunction. Sequence divergence between Asian and North American Gleditsia is much lower than between Asian and North American species of its closest relative, Gymnocladus. Estimates of Asian-North American divergence times for Gymnocladus are in general accordance with fossil data, but estimates for Gleditsia suggest recent divergences that conflict with ages of known North American Gleditsia fossils.

Phylogenetic relationships in Gleditsia (Leguminosae) based on ITS sequences

We used nucleotide sequences from the internal transcribed spacers and 5.8S gene of nuclear ribosomal DNA to test competing phylogenetic and biogeographic hypotheses in Gleditsia. Eleven of 13 Gleditsia species were sampled, along with two species of its sister genus, Gymnocladus. Analyses of ITS data and of a combined data set that included sequences of ITS and two chloroplast genes supported several conclusions that were interpreted in light of fossil data and current legume phylogeny. Gleditsia and Gymnocladus appear to have originated in eastern Asia during the Eocene. Eastern North American species of both genera most likely evolved from ancestors that migrated across the Bering land bridge, but the eastern Asian/eastern North American disjunction appears to be much older in Gymnocladus than in Gleditsia. Gleditsia amorphoides, from temperate South America, is sister to the rest of the genus, suggesting early long-distance dispersal from Asia. The remainder of Gleditsia is divided into three unresolved clades, possibly indicating a split early in the evolution of the genus. Two of those clades contain only Asian species, and one contains Asian and North American species. The North American species, Gleditsia triacanthos and Gleditsia aquatica, are polymorphic and paraphyletic with respect to their ITS and cpDNA sequences, which suggests recent diversification.

Landscape genetics of the key African acacia species Senegalia mellifera (Vahl)– the importance of the Kenyan Rift Valley

Acacias across Africa have enormous ecological and economic importance, yet their population genetics are poorly studied. We used seven microsatellite loci to investigate spatial genetic structure and to identify potential ecological and geographic barriers to dispersal in the widespread acacia, Senegalia (Acacia) mellifera. We quantified variation among 791 individuals from 28 sampling locations, examining patterns at two spatial scales: (i) across Kenya including the Rift Valley, and (ii) for a local subset of 11 neighbouring locations on Mpala Ranch in the Laikipia plateau. Our analyses recognize that siblings can often be included in samples used to measure population genetic structure, violating fundamental assumptions made by these analyses. To address this potential problem, we maximized genetic independence of samples by creating a sibship‐controlled data set that included only one member of each sibship and compared the results obtained with the full data set. Patterns of genetic structure and barriers to gene flow were essentially similar when the two data sets were analysed. Five well‐defined geographic regions were identified across Kenya within which gene flow was localized, with the two strongest barriers to dispersal splitting the Laikipia Plateau of central Kenya from the Western and Eastern Rift Valley. At a smaller scale, in the absence of geographic features, regional habitat gradients appear to restrict gene flow significantly. We discuss the implications of our results for the management of this highly exploited species.

Conservation genetics and evolutionary history of Gleditsia caspica: Inferences from allozyme diversity in populations from Azerbaijan

Gleditsia caspica is an endemic treespecies found in the endangered lowlandHyrcanian forests near the Caspian Sea insoutheastern Azerbaijan and northwestern Iran. Phylogenetic analyses show that G.caspica is a derivative of G. japonica,a widely distributed species of eastern Asia. Using allozyme markers to investigate geneticdiversity within two populations fromAzerbaijan, we discovered that a populationfrom a protected nature reserve was composedlargely of first-generation hybrids betweenG. caspica and G. triacanthos,which is native to eastern North America but isplanted as an ornamental shade tree inAzerbaijan. Hybrids exhibited higher levels ofself-fertilization and lower seed productionthan either parental species. In the secondpopulation, composed of pure G. caspica,11 of 31 loci scored were polymorphic, theaverage number of alleles per locus was 1.39,and gene diversity was 0.105. All diversityestimates were substantially lower than thosewe obtained from a small sample of G.japonica from South Korea and than estimatesfrom more extensive samples of South KoreanG. japonica and North American G.triacanthos published by other workers. Weconclude that (i) in addition to ongoingthreats from habitat conversion andfragmentation, G. caspica may also bethreatened in Azerbaijan by hybridization withG. triacanthos; and (ii) low variabilityin G. caspica populations is most likelynot a consequence of recent habitat loss andreductions in population size, but insteadreflects a long history of range contractionand endemism following isolation from G.japonica during the Pliocene or Pleistocene.