Research

Research

My research interests are in the ecological and evolutionary processes that shape adaptation and speciation. I am also interested in the application of, and interaction between, ecological, demographic and genetic processes in species conservation and ecological restoration.

Current research

Theoretical approaches to genetic rescue

Genetic rescue and the choice of appropriate source material have become prominent issues in conservation biology with the increasing need for ecological restoration and augmentation of small populations. For restoration genetics, where individuals are used to establish a new population, there are often many questions about how the genetic composition of the founders may influence establishment success (i.e., population persistence) and adaptation to local environmental conditions. Here we are using theory to examine how the composition of the founders influences restoration establishment and success. For population augmentation, this project will also test scenarios of how immigrant numbers, composite provenancing and rates of environmental change can influence population persistence. The goal here is to use theory to make predictions that can be tested by real world examples in animal and plant systems.

The role of mating system in the evolutionary dynamics of hybrid zones

Self-incompatibility (SI) is very common in flowering plants and is a genetically controlled mechanism to avoid inbreeding. In species with SI systems, negative frequency dependent selection results in a number of characteristics including high allelic diversity, even allele frequencies at equilibrium, increased effective migration rates and low population differentiation for S alleles. Given that rare S alleles have a selective advantage, for recently diverged species with incomplete reproductive barriers, the sharing of S alleles among species can facilitate introgression (adaptive introgression). The Antirrhinum genus contains several species with gametophytic SI that readily hybridize. We are interested in examining if the SI system facilitates introgression among closely related species by examining genetic structure and paternity in a hybrid zone of A. majus pseudomajus and A. majus striatum in the Pyrenees in Spain.

The evolution of novel S-alleles

An enduring puzzle in SI is how the high diversity observed in nature arises and how this is maintained. Based on the underlying recognition mechanism, SI can be classified into two main groups: self- and non-self recognition self-incompatibility. To date, most work has focused on diversification within self-recognition systems; this is despite expected differences in the evolutionary pathways and outcomes of diversification. Here, we use a deterministic population genetic model and stochastic simulations to investigate the pathways and conditions associated with novel S haplotype evolution in a non-self recognition (SRNase/S Locus F-box (SLF)) SI system. For this model the pathways for diversification involve either the maintenance or breakdown of SI and can vary in the order of mutations of the female (SRNase) and male (SLF) components. By examining diversification in an NSR SI system, this model extends our understanding of the evolution and maintenance of haplotype diversity observed in a recognition system common in flowering plants.

Previous research

1. Local adaptation, outbreeding depression and self-incompatibility in fragmented populations of Rutidosis leptorrhynchoides (Asteraceae)

The primary focus of my Ph.D. was examining local adaptation and outbreeding depression using Rutidosis leptorrhynchoides, a perennial, herbaceous plant distributed across south-eastern Australia. In an evolutionary context, local adaptation and outbreeding depression are central concepts in speciation, adaptive differentiation and the transition from micro to macro-evolution. In addition, for conservation biology, the issues of local adaptation, outbreeding depression and ‘genetic rescue’ are directly relevant to the genetic management of small populations and seed sourcing for revegetation and ecological restoration.

A central component of my PhD was experimentally determining the genetic mechanisms underlying outbreeding depression and population divergence. In this project I also examined the potential benefits of introducing new genetic material (through introducing new self-incompatibility alleles) for diploid and tetraploid populations.

I assessed local adaptation using a transplant experiment that involved 18 population pairs separated by a range of distances from 0.7 – 600 km. For each population pair, seed from both the home and foreign populations were planted into soil from the home population site and grown in a common climate representative of the home population (Pickup et al. 2012).

To examine outbreeding depression, F1 , F2 , F3 and control (within home population) progeny were generated for 12 of the 18 population pairs using a glasshouse crossing experiment. For the second and third generations, backcrosses to the home and foreign populations were generated to examine the two primary genetic mechanisms underlying outbreeding depression: (i) the dilution of genes associated with local adaptation (admixture analysis), and (ii) the disruption of co-adapted gene complexes through recombination (recombination analysis). F1 , F2 , F3 , backcrosses and control progeny were planted into soil from the home population sites and, as for the local adaptation experiment, grown in a common climate representative of the home populations (Pickup et al. (2013b)).

In these two studies I was interested if measures or surrogates of population divergence, including geographic, environmental or genetic distance between parental populations could predict local adaptation and outbreeding depression. For genetic distance, I examined differentiation for both quantitative traits (QST) and microsatellite markers (FST).

To assess the decrease in fertilization success associated with the loss of S-alleles in small populations of both diploid and tetraploid races, experimental crosses were undertaken for 24 diploid and 5 tetraploid populations. We found that small populations have reduced fertilisation success and that genetic rescue, by introducing new genetic material from other populations, is an important means of ameliorating mate limitation issues associated with reduced S-allele diversity in both diploid and tetraploid races (see Pickup and Young 2008 and Young and Pickup 2010).

2. Sex ratio variation in flowering plants

Understanding the mechanisms underlying sex-ratio variation in dioecious species is a key question in evolutionary biology. Deviations from the expected 1:1 sex ratio maintained by negative frequency-dependent selection indicate either a differential cost in rearing each sex or a difference in the survival of the sexes after parental investment. Although male bias is more common than female bias in dioecious species, female-biased sex ratios have been reported in species with heteromorphic sex chromosomes, suggesting that sex determination system may influence sex ratios. The two primary hypotheses to explain female-biased sex ratios include; (i) Certation, where gametophytic selection results from differential pollen tube growth of male and female determining microgametophytes, and (ii) Sex specific mortality, where the sexes show differential growth and/or survival after the period of parental investment.

Rumex hastatulus is a wind pollinated, dioecious annual herb distributed throughout mainland North America from Texas north to Illinois, and east through to North Carolina and north to New York. Rumex hastatulus provides an ideal study system to examine the potential role of sex chromosomes in determining variation in sex ratios since it has two distinct cytological races. Populations occurring from North Carolina to Florida and Mississippi (the North Carolina race) are the XX and XY1Y2 karyotype (2n = 8 in females and 2n = 9 in males), while population from Louisiana to Texas and Oklahoma (the Texas race) are the XX and XY karyotype (2n = 10 in both sexes).

This research aimed to answer the following questions:

• Is there female bias in populations of R. hastatulus?
• Are there differences in the patterns of sex-ratio variation between the two chromosome races?
• Does maternal mating environment influence female bias? Does this vary between the two chromosome races? (see Pickup and Barrett 2013)
• Does the degree of pollen competition influence progeny sex ratios? (see Field et al 2012)
• What is the role of frequency and density in sex ratio variation?
• Is there sexual dimorphism in R. hastatulus and does this vary across the life-cycle? (see Pickup and Barrett 2011)
• Is the geographic variation in sexual dimorphism among populations of the two chromosome races?

To further explore sex ratio variation in flowering plants, in collaboration with David Field, I used comparative analysis of 241 species of flowering plants (representing 124 genera and 61 families) to examine correlates of sex ratio variation in flowering plants. We investigate questions such as what is the frequency of biased sex ratios? What life history traits are associated with biased sex ratios? What is the role of sex-determining system and non-equilibrium conditions in biased sex ratios? Our study indicates that gender-based differences in the cost of reproduction, sex determining mechanisms and non-equilibrium conditions each play important roles in affecting flowering sex ratios in dioecious plant species (see Barrett et al. 2010; Field et al. 2013a,b).