Two current research projects on a thalloid liverwort Marchantia inflexa Nees et Mont. (phylum Marchantiophyta):
This species has separate male and female plants (dioecious) and grows on boulders along rivers from the southern United States to northern Venezuela. Both sexes produce specialized asexual offspring. In this system, the foci are on investigating tradeoffs (among growth, asexual reproduction and sexual reproduction), sex-specific demography patterns, and population dynamics that are important in maintaining sexual reproduction.
The projects using M. inflexa include:
1) Population sex ratio in clonal organisms: causes and consequences in natural and novel habitats
The population sex ratios are assumed to be related to sex (male/female) differences in life history traits, such as asexual reproduction (Eppley et al 1998; Obeso et al. 1998). However, this linkage has not been systematically examined. We use a species with spatially structured populations that exhibit the full range of population sex ratios. Marchantia inflexa has recently invaded a human manipulated habitat, and experienced an altered spatial structure and very different environmental conditions relative to its historical habitat (Groen et al. 2010). These new populations show an evolutionary response in their life history patterns (Brzyski et al. in press). We have been quantifying the different sex ratios (genetic/population) and testing predicted relationships between them. We are also determining the genetic structure of the populations to assess the dynamics of these spatially structured populations, including populations in the novel habitats. Finally, we plan to test for congruence between patterns in asexually reproduction of selected genotypes, measured under controlled conditions, and observed field patterns of those same genotypes.
2) Variation in sexual reproductive traits within populations: genetic or environmental causes
A common expectation in nature is that two individuals experiencing the same environment will express the same phenotype (physiology/morphology). If they differ, then we assume a genetic difference underlies the phenotypic differences. In plants, one such phenotypic variation is the presence of sexual reproductive traits in individuals and the absence of these traits in nearby individuals (Pohjamo et al. 2008, Korpelainen et al. 2005). This pattern of variation is particularly interesting because sexual reproduction requires some synchrony between mates to ensure success. While within a species, individuals can differ in their sensitivity to the environmental cues to initiate sexual reproduction this difference is generally observed to exist across populations within a species range (Eckert 2002) not within a population. Even in species with asynchronous reproduction, as is the case for some plants that reproduce year round, individuals overlap in the timing of reproduction and most importantly there is, to our knowledge, no evidence of variation in sensitivity to environmental cues within a natural population.
From long-term studies of the study plant species, Marchantia inflexa, we observed populations that contain both sex-expressing and non sex-expressing individuals at the same time, and where some populations infrequently or never express sex (McLetchie and Puterbaugh, 2000; Fuselier and McLetchie, 2004). One possible explanation for sex expression variation within a population is subtle spatiotemporal variation in environmental cues and in some cases populations not experiencing the necessary environmental cues for initiating sexual reproduction. Another possibility is there are genetic differences controlling how plants respond to environmental cues resulting in the observed spatiotemporal patterns of sex expression. In the study species support for a genetic cause for sex expression was recently detected (Brzyski et al. in press).
This project focuses on within population patterns and is determining if variation in sex expression is genetic or due to environmental variation and is testing whether reproduction mode is associated with levels of genetic diversity.
Brzyski, J. R., W. Taylor, D. N. McLetchie, in press, Reproductive allocation between the sexes, across natural and novel habitats, and its impact on genetic diversity. Evolutionary Ecology
Eppley SM, Stanton ML, Grosberg RK 1998. Intrapopulation Sex Ratio Variation in the Salt Grass Distichlis spicata The American Naturalist, Vol. 152: 659-670
Fuselier, L. and McLetchie, D. N. 2004. Microhabitat and sex distribution in Marchantia inflexa, a dioious liverwort. The Bryologist 107: 345-356.
Groen, K. E., Stieha, C., Crowley, P. H., and McLetchie, D. N. 2010 Sex-specific plant responses to light intensity and canopy openness: implications for spatial segregation of the sexes. Oecologia 162: 561-570 doi:10.1007/s00442-009-1473-z
Korpelainen, H., Pohjamo, M., Laaka-Lindberg, S., 2005. How efficiently does bryophyte dispersal lead to gene flow? Journal of the Hattori Botanical Laboratory 97, 195–205.
McLetchie, D. N. and Puterbaugh, M. N. 2000. Population sex ratios, sex-specific clonal traits and tradeoffs among these traits in the liverwort, Marchantia inflexa. Oikos 90: 227-237.
Obeso JR, Alvarez-Santullano M, Retuerto R 1998 Sex ratios, size distributions, and sexual dimorphism in the dioecious tree Ilex aquifolium (Aquifoliaceae) American journal of Botany 85:1602-1608
Pohjamo M, Korpelainen H, Kalinauskaite´ N (2008) Restricted gene flow in the clonal hepatic Trichocolea tomentella in fragmented landscapes. Biological Conservation 141:1204–1217