Conservation genetics of epiphytic and terrestrial orchids

We have used terrestrial and epiphytic orchids as model systems to investigate the processes influencing the evolution of fine-scale, spatial genetic structure within populations and how this structure may be altered by various factors, including human-mediated disturbance. As orchid seeds are minute, dust-like, and wind-dispersed, it is has generally been assumed that they would be dispersed over long distances. Despite this long-standing expectation, our papers were among the first to show that orchid populations may exhibit significant fine-scale genetic structure (FSGS): structure that is indicative of seed dispersal that is at least partially restricted about the maternal plant. To date we have studied FSGS in six orchid species, one epiphytic (Laelia; Trapnell et al.

Graphical population genetics - the geometry of genetic structure

In collaboration with Dr. Rodney Dyer (Virginia Commonwealth U.) we have developed population graphs, a novel network-based procedure for quantifying patterns of inter-population gene flow (Dyer & Nason 2004; Dyer et al. 2010). While most population geneticists recognize that gene flow is probably best represented in the form of interacting networks of populations, in boiling down this complexity to one or a few summary statistics (e.g., Fst) most of our analytical procedures fail to capture the spatially-explicit connectivity among populations that we are often most interested in. A population graph, in contrast, provides a formal statistical model of the network of the effective gene flow - the genetic connectivity - among populations.

The ecology and evolution of figs and fig wasps

Although figs (genus Ficus) are a diverse and a common component of tropical and subtropical ecosystems, individual species often occur at extraordinarily low population densities, creating a challenge for successful fig wasp dispersal between hosts and pollination. Working in Central America, we have used genetic markers to show that, despite their small size and short life spans, fig wasp pollinators routinely disperse >10 km between host trees and that interbreeding populations of figs comprise many hundreds to thousands of individuals (Nason et al. 1996; Nason & Hamrick 1997; Nason et al. 1998). We have also used molecular methods to show that the rate of pollinator-mediated gene flow greatly exceeds that of bat- and bird-mediated seed gene flow (Yu et al.

The ecology and evolution of invasive plant species

Combining ecological and genetic approaches, we have examined the process of biological invasion in three different plant systems. In multiflora rose (Rosa multiflora) we used genetic markers to quantify the relative contributions of sexual reproduction via seed versus vegetative reproduction via clonal spread to local patterns of invasion (Jesse et al. 2010). In purple loosestrife (Lythrum salicaria) we compared phenotypic plasticity of native European versus invasive North American populations in response to variation in water and nutrient levels (Chun et al. 2007). This research included development of a new, two-stage multivariate approach to test two important attributes of multivariate vectors of phenotypic change: the magnitude and direction of mean trait differences between environments.

The evolution of fine-scale spatial genetic structure

In early papers (Loiselle et al. 1995; Kalisz et al. 2001), we introduced a measure of inter-individual genetic relatedness that has become the default in the most-widely-used software for the spatial autocorrelation analyses of fine-scale genetic structure (FSGS) within populations. We have used this measure, a kinship coefficient (denoted Fij), in a number of studies investigating the various factors influencing dispersal and evolution of genetic structure within plant populations, including life-stage effects (Kalisz et al. 2001; Chung et al. 2003), vegetative reproduction and clonal spread (Chung et al. 2004a), inbreeding (Chung et al. 2004b, 2005a, Chung & Nason 2007), a population's successional stage (Chung et al. 2007; Chung et al. 2011), disturbance history (Parker et al. 2001; Chung et al.

The evolution of host race formation and cryptic speciation in plant-feeding insects

The exceptional diversity of phytophagous insects may be due in part to their propensity for speciation via host-race formation. In our research, we have focused on two closely-related goldenrods (Solidago altissimaand S. gigantea) and their insect herbivores as a model system to study host shifts (and eventual speciation) by multiple evolutionarily independent insect lineages on the same host plant pair. Our ecological and genetic analyses establish host-race formation as an important mechanism of diversification in phytophagous insects (Nason et al. 2002b; Stireman et al. 2005) and their insect parasitoids (Stireman et al. 2006; Kolaczan et al. 2009).

The evolution of population genetic and phylogeographic structure

We have combined traditional population genetic approaches with coalescent and graphical modeling techniques to gain insight into how deep-time geological and shallow-time climatic changes have influenced species distributions and the spatial organization of genetic variation. Mexico’s Baja California Peninsula is for us a model landscape in which to study large-scale patterns of spatial genetic structure. Numerous studies have used mtDNA sequences to quantify this structure in vertebrates, extrapolating common signals of deep-time vicariance events to other organisms.