David L. Field, Postdoctoral Fellow

Department of Ecology and Evolutionary Biology University of Toronto
25 Willcocks Street
Toronto, Ontario, Canada
M5S 3B2
Phone: (416) 978-5603
Email: david.field@utoronto.ca

Degrees:

B.Sc. (Honours)

(2001) Department of Biological Sciences, University of Wollongong, Australia. Thesis Title: Local Plant Density and its Impact on the Mating System of Persoonia bargoensis. Advisors: Profs. David Ayre (UOW), Robert Whelan (UOW).

Ph. D.

(2008) Commonwealth Scientific & Industrial Research Organisation (CSIRO) – Division of Plant Industry, Canberra & Department of Biological Sciences, University of Wollongong (UOW), Australia Thesis title: The importance of ecological factors in determining the patterns of interspecific hybridisation in fragmented landscapes of Eucalyptus aggregata. Advisors: Profs. Andrew Young (CSIRO), David Ayre (UOW), Robert Whelan (UOW).

Employment History

Postdoctoral Fellow

(Nov 2007 – June 2008) CSIRO – Plant Industry. With Andrew Young and Linda Broadhurst. Research topic: development of new techniques to measure inter-population gene flow in autopolyploids and applying these to assess the gene flow dynamics in matrix habitats of the autohexaploid Eremophila glabra in central Australia.


Postdoctoral Fellow, University of Toronto


(July 2008 – present) Professor Spencer Barrett’s lab, in the Department of Ecology and Evolutionary Biology (EEB) at the University of Toronto, Canada.

 

RESEARCH INTERESTS

 

I am a plant evolutionary biologist interested in the processes that lead to and maintain separate species, the mechanisms responsible for transitions in reproductive strategies, and the limits to local adaptation. This comes from a desire to explain the remarkable diversity of strategies that plants have evolved to survive and reproduce in their local environment. I am also interested in applying evolutionary theory to practicle problems in order to predict how populations will survive and adapt in rapidly changing environments.

Some of the questions I work on include:

  • What causes variation in rates of hybridization between distinct species and what are the evolutionary consequences?
  • What mechanims are responsible for evolutionary transitions in plant reproductive strategies?
  • What mechanisms underly sex ratio variation?
  • What processes constrain evolution and local adaptation at species range limits?
  • How important is polyploidy in ecological divergence?

My research integrates different approaches including large-scale geographic surveys, manipulative field experiments, greenhouse experiments, genetics, program development, theory and comparative analysis. Currently, I am a Postdoctoral Fellow with Professor Spencer Barrett at the University of Toronto, Canada.

A print friendly CV is available here (pdf) and for more information on my research see my web pages on Google.


CURRENT RESEARCH

1. Evolution and maintenance of separate sexes

The evolutionary transition from hermaphroditism (combined sexes) to dioecy (co-occurrence of males and females) has occurred independently at least 100 times in the flowering plants. However, in many species, a low to moderate frequency of labile hermaphrodites can remain, with some populations consisting of males, females and hermaphrodites. This condition is typically referred to as subdioecy or trioecy. This tendency for hermaphrodites to not be entirely replaced by male phenotypes raises the question of whether this state represents a stable sexual system and what role gender plasticity plays in hindering the transition to full dioecy. In the Barrett lab, I am using manipulative greenhouse experiments to investigate the maintenance of trioecy using Sagittaria latifolia, an aquatic species common in N. America with some populations consisting of all three sex phenotypes (males, females, hermaphrodites- pictured). I am also using theoretical models to investigate the role that gender plasticity may play in both facilitating the evolutionary transition to dioecy and the maintenance of three sexes.



Inflorescences of hermaphrodite, female and male plants of Sagittaria latifolia

2. Sex ratio variation in flowering plants

Theory predicts that when the cost of producing males and females is equal, a 1:1 sex ratio should be maintained by negative frequency-dependent selection. However, there is evidence from surveys of plant populations of frequent departures from equality. In the Barrett lab in collaboration with PDF Melinda Pickup, I am investigating this problem with both modeling approaches and a meta-analysis of the literature to determine how often bias occurs and to identify the ecological and life history correlates of male and female bias. Initial modelling results have demonstrated the importance of non-equilibrium conditions and life history as explanations of sex ratio variation (Barrett et al. 2010 Phil Trans Roy Soc).



Male and female plant of the wind pollinated herb, Rumex hastatulus

3. Mechanisms governing female biased sex ratios

Whereas the causes of male biased sex ratios have been well studied, the mechanisms behind female biased sex ratios remain poorly understood. For species with sex chromosomes, competition between female- and male-determining microgametophytes (pollen grains) has been proposed as one of the causes of female biased sex ratios (certation hypothesis). This is thought to be due to Y-chromosome degeneration resulting differential performance of male and female determining pollen grains, resulting in preferential fertilization by female-determining pollen grains. In the Barrett lab, in collaboration with PDF Melinda Pickup, I am exploring the mechanisms governing female bias in Rumex hastatulus, an annual species native to the southern USA ranging in distribution from Texas to North Carolina. This is an interesting species which has two distinct chromosome races, the North Carolina karyotype (females = XX, 2n = 8; males = XY1Y2; 2n = 9) and the Texas karyotype (females XX, males XY, 2n = 10). Using controlled crosses we have found significant female biased progeny arrays (sex ratio = 0.62) with the probability of producing female offspring increasing with pollination intensity. We are investigating this relation further by examining the importance of plant density and local sex ratio on progeny sex ratios in manipulated experimental arrays at the University of Toronto field station (pictured top right).

 



Setting up experimental arrays of male and female Rumex hastatulus



Female flower of Rumex hastatulus 20x magnification (left) and pollen tubes competing to fertilize a single ovule 100x (right)



4. Hybridization and the maintenance of distinct species

My PhD research (2002-2008) with Prof. Andrew Young (CSIRO), Prof. David Ayre and Prof. Rob Whelan University of Wollongong) examined the mechanisms governing patterns of hybridization and the barriers to gene flow in the genus Eucalyptus. Many species pairs exhibit dramatic variation in hybrid frequency at both the individual and population level however, surprisingly few studies have examined the basis of this variation. This work used a combination of field surveys, population genetics, and manipulative experiments to examine the processes governing variation in hybridization between the uncommon E. aggregata, and the more abundant E. rubida and E. viminalis (pictured).

I began with large-scale geographic surveys, morphological and molecular markers to obtain the first evidence of extensive hybridization between these three distinct species (Field et al 2009 Conservation Genetics). Further work uncovered evidence that the relative abundance of species is an important parameter determining the frequency of hybrid formation, seed production and seedling performance (Field et al 2008 Journal of Ecology).

I also used SSR (microsatellites) markers, and analyses of fine-scale demographic and genetic structuring in several hybrid zones of E. aggregata and E. rubida. I uncovered evidence of asymmetrical gene flow from the common towards the rare species, which likely reflects a combination of demography and differences in style lengths (E. rubida: ~7 mm, E. aggregata: ~4mm) preventing pollen tubes of smaller-flowered species from fertilizing larger-flowered species (Field et al 2011 Heredity).

In a following study, I used SSR markers and parentage analysis in the same hybrid zones, and showed that large-scale landscape process, local demography and pre-mating phonological barriers can govern frequencies of hybrid formation at an individual level in a predictable way (Field et al 2011 Molecular Ecology*) [*Featured in upcoming perspective article by Berthold Heinz].

Hybrid breakdown can also manifest at early life stages through increased susceptibility to herbivores, potentially due to the breakup of defence chemicals between species with contrasting leaf chemistry. In collaboration with Dr. Rose Andrew (currently a PDF at UBC, Vancouver), we compared the susceptibility to attack by natural herbivores (Christmas beetles) and the effects of hybridization on leaf chemical defences of hybrid seedlings to purebreds (manuscript in preparation).





Black gum Eucalyptus aggregata (left) & Candlebark - E. rubida (right)



Purebred (A,R,V) and hybrid (AR, AV) seedling morphology



Beetles are a primary herbivore on these Eucalyptus (top) and can cause severe leaf damage (bottom)

5. Polyploid population genetics

Polyploidy is common throughout much of the plant kingdom and is also present in a range of animals. It is well recognized that both allo- and auto-polyploidy are of major evolutionary and ecologically importance in plants. As a consequence there has been increasing interest in investigating the ecological and genetic attributes of polyploids and developing our understanding of its role in plant evolution and importance in biological conservation. However, this has not translated to the availability of analytical tools for population genetic analysis for polyploids. The development of population genetic software for polyploid organisms have been hampered by the complexities of polyploid inheritance and difficulties in interpreting molecular marker data. I am bridging this gap with development of new theoretical approaches and associated software programs for a range of population genetic analyses. In collaboration with Prof. Andrew Young, Dr. Linda Broadhurst at CSIRO Plant Industry (Australia) and with Dr. Anders Larsen (Denmark), I am also testing these new approaches with real genetic marker data in both tetraploid and hexaploid populations (Adansonia digitata and Eremophila glabra, respectively). Auto-polyploids pose a number of significant challenges for population genetic analyses including polsomic inheritance and double reduction, and genotype uncertainty with codominant markers

 



Auto-polyploids pose a number of challenges for population genetic analyses due to polysomic inheritance and double reduction, and genotype uncertainty when using codominant markers

 

Teaching Experience

Jan 2009- May 2009

Lecturer and Course Coordinator: Dept. of Ecology & Evolutionary Biology, University of Toronto, Canada. 2nd year course “Evolution and Adaptation”

2002- 2008

Undergraduate lab tutor: Dept. Biology, University of Wollongong: 1st to 3rd year undergraduate biology - BIOL103 Molecules, Cells and Organisms, BIOL240 Functional Biology of Plants and Animals, BIOL351 Conservation Biology: Marine and Terrestrial Populations

2006

Tutor, Australian National University, School of Resources, Environment & Society, Canberra (2006) - Masters coursework students in population genetics.

2004

Undergraduate lab tutor: Dept. Biology, University of Wollongong: 1st to 3rd year undergraduate biology - BIOL103 Molecules, Cells and Organisms, BIOL240 Functional Biology of Plants and Animals, BIOL351 Conservation Biology: Marine and Terrestrial Populations

2002- 2008

National Youth Science Forum (2004) - Canberra, Australia

2004

CSIRO Student Research Scheme (2004) – My students won prize for best poster presentation for 2004

 

Reviewer Experience


American Naturalist, Molecular Ecology, Evolutionary Applications, Conservation Biology, Heredity, International Journal of Plant Sciences, Oecologia, New Phytologist, Conservation Genetics, Australian Journal of Botany, Austral Ecology.

PUBLICATIONS

  1. *Field DL, Ayre DJ, Whelan RJ, Young AG (2011) The importance of pre-mating barriers and the local demographic context for contemporary mating patterns in hybrid zones of Eucalyptus aggregata and E. rubida. Molecular Ecology, doi: 10.1111/j.1365-294X.2011.05054.x [* featured perspective article by Berthold Heinz] (pdf)

  2. Field DL, Ayre DJ, Whelan RJ, Young AG (2011) Patterns of hybridization and asymmetrical gene flow in hybrid zones of the rare Eucalyptus aggregata and common E. rubida. Heredity, doi:10.1038/hdy.2010.127 (pdf)

  3. Barrett SCH, Yakimowski SB, Field DL, Pickup M (2010) Ecological genetics of sex ratios in plant populations. Philosophical Transactions of the Royal Society B, 365, 2549-2557 (pdf)

  4. Field DL, Ayre DJ, Whelan RJ, Young AG (2009) Molecular and morphological evidence of natural interspecific hybridization between the uncommon Eucalyptus aggregata and the widespread E. rubida and E. viminalis. Conservation Genetics, 10, 881-896. (pdf)

  5. Field DL, Ayre DJ, Whelan RJ, Young AG (2008) Relative frequency of sympatric species influences interspecific hybridization rates, seed production and seedling performance in fragmented populations of Eucalyptus aggregata. Journal of Ecology, 96, 1198-1210. (pdf)

  6. Field, DL, Ayre, DJ and Whelan, RJ (2005). Local plant density and its impact on the pollinator behaviour and the breeding system of Persoonia bargoensis. International Journal of Plant Sciences, 166, 969-977. (pdf)

Publications in preparation

  • Field DL, Pickup M, Barrett SCH (in prep) Sex ratio variation in flowering plants.

  • Field DL, Pickup M, Barrett SCH (in prep) The influence of stigmatic pollen load on fertilization success, sex ratio and fitness in a wind pollinated annual dioecious plant.

  • Field DL, Barrett SCH (in prep) The coexistence of males, females and hermaphrodites.

  • Field DL, Barrett SCH (in prep) Sex allocation strategies in hermaphrodite, dioecious and subdioecious populations of an aquatic herb.

  • Pickup M, Field DL, Barrett SCH (in prep) The influence of frequency and density on progeny sex ratios in an annual dioecious plant.

  • Field DL, Andrew RL, Foley W, Young AG (in prep) Increased hybrid susceptibility to insect herbivory in Eucalyptus.

  • Field DL, Broadhurst LM, Young AG (in prep) Population assignment in autopolyploid populations.

  • Field DL, Broadhurst LM, Young AG (in prep) POLYASSIGN: A program for population assignment testing for autopolyploids using multilocus genotype or phenotype information.


Conference papers presented

  • Field DL, Barrett SCH. 2010. The coexistence of males, females and hermaphrodites in flowering plant populations. SSE meeting, Portland, U.S.A

  • Field D.L., Young A., Ayre D.J. and Whelan R. 2007. Swamping out rare genes: habitat fragmentation promotes hybridization in a woodland Eucalypt. SSE meeting, Christchurch, New Zealand.

  • Field D.L., Young A., Ayre D.J. and Whelan R. 2005. Swamping the swamp gum: Habitat fragmentation increases hybridization in a woodland Eucalypt. Proceedings of 17th International Botanical Congress, Vienna.

  • Field D.L., Young A., Ayre D.J. and Whelan R. 2005. Hybrid Happy or Hardly Hybridising. Molecular Biology and Evolution and the Annual conference for the Genetic Society of Australasia, Auckland.

  • Field D.L., Young A., Ayre D.J. and Whelan R. 2004. Swamping the swamp gum: Habitat fragmentation and disturbance promotes hybridization in Eucalyptus aggregata. Proceedings of the 51st Annual conference for the Genetic Society of Australasia, Melbourne.
    Mayo Prize ($500). Best talk by a PhD student.

  • Field D.L., Young A., Ayre D.J. and Whelan R. 2003. The role of habitat fragmentation and disturbance in promoting hybridization in Eucalyptus aggregata. Proceedings of the 19th International Conference of Genetics, Melbourne, Australia.

  • Field, D.L., Ayre, D.J. and Whelan, R.J. 2001. Local plant density and its impact on the mating system of Persoonia bargoensis. Proceedings of the Ecological Society of Australia Conference 26-28th September, Wollongong, Australia.


 Fellowships and Awards

  • 2007               Conference bursary, ICB, Uni. of Wollongong ($400)

  • 2005               Conference bursary, ICB, Uni. of Wollongong ($500)

  • 2005               Research travel bursary, Uni of Wollongong ($500)

  • 2005               Molecular Biology & Evolution Conference student travel bursary ($500)

  • 2004               Mayo Prize ($500). Best talk by a PhD student, 51st Annual Meeting of Genetics Society of Australia, Melbourne

  • 2003               Free registration, International Congress of Genetics, Melbourne ($250)

  • 2003-2006      CSIRO Scholarship Top-up ($5,300 annual - 3 years)

  • 2002-2005      University Postgraduate Research Award University of Wollongong

  • 2001               Dean’s Merit List for academic performance

 

Referees

  • Professor Spencer Barrett. Department of Ecology and Evolutionary Biology, University of Toronto, Canada. Phone +1 416 978 4151, email: spencer.barrett@utoronto.ca (Postdoctoral Advisor)

  • Professor Andrew Young. CSIRO Plant Industry, Black Mountain Laboratories, ACT 2601. Phone +61 2 62465318, email: andrew.young@csiro.au (PhD advisor).

  • Professor David Ayre. Department of Biological Sciences, University of Wollongong. Phone +61 2 42213440, email: david_ayre@uow.edu.au (PhD & Honours advisor).