Caroli de Waal, M.Sc. Student
Department of Ecology and Evolutionary Biology
University of Toronto
25 Willcocks Street
Toronto, Ontario, Canada
Phone: (416) 978-5603
B.Sc. Plant Molecular Biology
(2004-2006) University of the Free State, Bloemfontein, South Africa
B.Sc. Hons. Botany.
(2007) University of the Free State, Bloemfontein, South Africa
Thesis title: Breeding Systems in African Lycium (Solanaceae)
(2010) Department of Ecology and Evolutionary Biology
Thesis title: Reproductive Ecology of bird-Pollinated Babiana (Iridaceae): Floral Variation, Mating Patterns and Genetic Diversity
- Connaught Scholarship, University of Toronto, 2008
- Dean’s Medal, best Honours degree in the Faculty of Natural and Agricultural Sciences, University of the Free State, 2007
- PPS Prize, best B.Sc. Honours student, University of the Free State, 2007
- Botanical Society of South Africa Award, best honours student in Botany, University of the Free State, 2007
CONFERENCE PAPERS PRESENTED
- De Waal, C., Venter, A.M. & Miller, J.S. Breeding systems of two South African Lycium species (Solanaceae). Annual conference of the South African Association of Botanists. January 2008, Drakensville, South Africa.
EXTRACURRICULAR ACADEMIC EXPERIENCE
- Graduate Teaching Assistant (2008), first year biology labs, University of Toronto
- Graduate Teaching Assistant (2007), second and third year botany labs, third year botany field excursion, third year mycology labs, University of the Free State, South Africa
- Teaching Assistant (2006-2007), first year biology labs, University of the Free State, South Africa
SUMMARY OF B.SC. HONS. THESIS
The cosmopolitan genus Lycium (Solanaceae) consists of ca. 80 species. Lycium species vary in sexual function, with many species being hermaphroditic, and some species either gynodioecious (Miller & Venable, 2002, 2003) or dioecious (Venter, 2000, 2007). The genus is an interesting system for the study of the evolution of sexual dimorphism, since atypically it has occurred in the presence of self-incompatibility (Miller & Venable, 2002). Miller and Venable (2000, 2002) addressed this relatively uncommon association by proposing that separate sexes evolved in Lycium following polyploidy which caused the breakdown of self-incompatibility, inbreeding depression and the conditions favouring the evolution of gender dimorphism.
Recent phylogenetic studies of the genus (Levin & Miller, 2005; Levin et al., 2007) provide support for a South American origin of the genus. Self-incompatibility is ancestral in the genus and phylogenetic studies suggest a single dispersal event for Lycium from the New to the Old World, since Old World Lycium species are monophyletic (Levin & Miller, 2005; Levin et al., 2007). However, Baker’s Law (Baker, 1955, 1967) states that a new coloniser would more likely be self-compatible than self-incompatible. We therefore expect to find that the establishment of the genus in Southern Africa was facilitated by a loss of self-incompatibility in the first colonists. Self-incompatibility would then be expected to be absent in Southern African Lyium, since a complex trait such as self-incompatibility can be lost relatively easily, but its loss is essentially irreversible (Igic et al., 2003).
To determine the compatibility status of species in the monophyletic African Lycium, I studied the summer-flowering L. cinereum Thunb. and the winter-flowering L. hirsutum Dunal, representing two different clades of the genus. I compared fruit and seed set following controlled cross- and self-pollination through field investigations and determined the presence or absence of self-incompatibility. I also investigated pollen-tube growth in L. hirsutum using fluorescence microscopy.
My results demonstrate that both L. cinereum and L. hirsutum are self-incompatible, based on fruit and seed production in controlled crosses and observations of pollen-tube growth. This in one of the first reports of self-incompatibility in Southern African Lycium species. Self-incompatibility has also recently been reported in L. ferocissimum and L. pumilum (Miller et al., 2008). Collectively these results indicate that self-incompatibility was maintained in African Lycium after long-distance dispersal from the New World and provides an interesting exception to Baker’s Rule (1955, 1967).
Baker, H. G. 1955. Self-compatibility and establishment after “long-distance” dispersal. Evolution 9:347–349.
Baker, H.G. 1967. Support for Baker’s Law—as a rule. Evolution 21:853–856.
Igic, B., L. Bohs, and J. R. Kohn. 2003. Historical inferences from the self-incompatibility locus. New Phytol. 161:97–105.
Levin, R. A. and Miller, J.S. 2005. Relationships within tribe Lycieae (Solanaceae): paraphyly of Lycium and multiple origins of gender dimorphism. Am. J. Bot. 92:2044–2053.
Levin, R. A., Shak, J.R., Bernardello, G., Venter, A.M. and Miller, J.S. 2007. Evolutionary relationships in tribe Lycieae (Solanaceae). Acta Hort. 745:225–239.
Miller, J. S. and Venable, D.L. 2000. Polyploidy and the evolution of gender dimorphism in plants. Science 289:2335–2338.
Miller, J.S. and Venable, D.L. 2002. The transition to gender dimorphism on an evolutionary background of self-incompatibility: an example from Lycium (Solanaceae). Am. J. Bot. 89:1907–1915.
Miller, J.S. and Venable, D.L. 2003. Floral morphometrics and the evolution of sexual dimorphism in Lycium (Solanaceae). Evolution 57:74–86.
Miller, J.S., Levin, R.A. and Feliciano, N.M. 2008. A tale of two continents: Baker’s Rule and the maintenance of self-incompatibility in Lycium (Solanaceae). Evolution 62-5:1052-1065.
Venter, A. M. 2000. Taxonomy of the genus Lycium L. (Solanaceae) in Africa. Ph.D. thesis, Univ. of the Orange Free State, Bloemfontein, South Africa.
Venter, A.M. 2007. Lycium hantamense (Solanaceae), a new species from the Hantam–Roggeveld Centre of Plant Endemism, South Africa. S. Afr. J. Bot. 73:214–217.
I am generally interested in plant evolutionary biology, particularly evolutionary ecology and ecological and evolutionary genetics. I am specifically interested in the ecology and evolution of plant mating systems, gender strategies, pollination biology and evolutionary transitions in flowering plant reproductive systems.
The evolution of sex ratios in flowering plants
I am interested in the mechanisms governing sex ratios in dioecious flowering plants, particularly those related to female-biased sex ratios. Sex ratios often deviate from 1:1, with male bias being more common than female bias (Delph, 1999). Various mechanisms can influence sex ratios, and to understand the evolution of sex ratios we need to determine when and how biases are established during the life cycle of the plant (Lloyd & Webb, 1977; Bierzychudek & Eckhart, 1988; Ågren et al., 1999; Taylor, 1999; De Jong & Klinkhamer, 2002; Taylor & Ingvarsson, 2003).
Some of the main hypotheses that have been proposed to account for female-biased sex ratios include 1) certation (Correns, 1922) – the selective fertilization that result from differential pollen-tube growth of female- versus male-determining microgametophytes; 2) gender-specific mortality, i.e. differences in the performance and viability of the sexes after parental investment (Zarzycki & Rychlewski, 1972), and 3) sex ratio distorters (Taylor, 1999).
Female-biased sex ratios, although infrequent, are often reported in species with heteromorphic sex chromosomes (Lloyd, 1974). This suggests a relation between the sex determination system and either the performance of the sexes, or the performance of the sex-chromosomal genotype of gametophytes.
Sex ratios in Rumex
Rumex (Polygonaceae) is a useful genus to study mechanisms governing sex ratios, because some of the wind-pollinated dioecious species in the genus exhibit female-biased sex ratios. These species are also characterized by two distinct sex-determining mechanisms and sex-chromosome systems. In some Rumex species sex is determined by an XX/XY chromosome system, with XX being the female genotype and XY the male genotype (Löve, 1944; Smith, 1969). In other species, such as R. nivalis, a more complicated XX/XY1Y2 sex-chromosome system occurs (e.g. Smith, 1969). R. hastatulus is unique in being polymorphic for its sex-determining system with both an XX/XY and an XX/XY1Y2 sex-chromosome system.
R. hastatulus is a wind-pollinated, dioecious annual herb that occurs on coastal plains and disturbed areas in sandy soils throughout south-eastern United States (Figure 1). Populations occurring from North Carolina to Florida and Mississippi (the North Carolina race) are characterized by the XX/XY1Y2 sex-determining system (2n = 8 in females and 2n = 9 in males), whereas populations that occur from Louisiana to Texas and Oklahoma (the Texas race) possess the XX/XY sex-determining system (2n = 10 in both sexes). A third chromosome race, termed the southern Illinois-Missouri race, is mentioned by Smith (1969).
As part of my PhD project, I aim to explore the following topics pertaining to sex ratios in Rumex hastatulus: 1) Sex ratio variation between the different chromosomes races, and the possible association between sex ratio and population size and/or density; 2) The sex ratios of open-pollinated seed arrays, and the effect of the maternal mating environment; 3) Mechanisms that could result in biased sex ratios, such as certation, sex-specific mortality, and sex ratio distorters.
Figure 1. a. Rumex hastatulus in its natural habitat. b. Winged fruits on a female R. hastatulus plant. c. Male R. hastatulus plant. (Images from University of South Carolina Upstate herbarium database.)
Ågren, J., Danell, L., Elmquist, T., Ericson, L., Hjälten, J. 1999. Sexual dimorphism and biotic interactions. In: Geber, M.A., Dawson, T.E., Delph, L.F., eds. Gender and sexual dimorphism in flowering plants. Berlin, Gemany: Springer, 149-174.
Bierzychudek, P. and Eckhart, V. 1988. Spatial segregation of the sexes of dioecious plants. American Naturalist 132: 34-43.
Correns, C. 1922. Sex determination and numerical proportion of genders in Common Sorrel (Rumex acetosa) (Translated from German). Biol. Zentralblatt 42:465-480.
De Jong, T.J. & Klinkhamer, P.G.L. 2005. Evolutionary ecology of plant reproductive strategies. Cambridge University Press, Cambridge, UK.
Delph, L. F. 1999. Sexual dimorphism in life history. In M. A. Gerber, T. E. Dawson, and L. F. Delph, editors. Gender and sexual dimorphism in flowering plants. Springer, Berlin, Germany, 149-174.
Lloyd, D. G. 1974. Female-predominant sex-ratios in angiosperms. Heredity 32:35-44.
Lloyd, D.G. and Webb, C.J. 1977. Secondary sex characters in plants. Botanical Review 43: 177-216.
Löve, A. 1944. Cytogenetic studies on Rumex subgenus Acetosella. Hereditas 30: 1-136.
Smith, B.W. 1969. Evolution of sex determining mechanisms in Rumex, pp. 1972-182. In Darlington, C.D. and Lewis, K.R., eds. Chromosomes Today. Vol. 2. Plenum Press, New York.
Taylor, D.R. 1999. Genetics of sex ratio variation among natural populations of a dioecious plant. Evolution 53: 55–62.
Taylor, D.R. & Ingvarsson, P.K. 2003. Common features of segregation distortion in plants and animals. Genetica 117: 27–35.
Zarzycki, K., and Rychlewski, J. 1972. Sex ratios in Polish natural populations and in seedling sampling of Rumex acetosa L. and R. thyrsiflorus Fing. Acta Biologica Cracoviensia Series Botanica 15:135-151.
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