Jon Lovett Doust

Sexual specialization and inbreeding avoidance in the evolution of dioecy

Dioecy has evolved independently, many times, among unrelated taxa. It also appears to have evolved along two contrasting pathways: (1) from hermaphroditism via monoecy to dioecy and (2) from hermaphroditism via gynodioecy to dioecy. Most dioecious plants have close cosexual relatives with some means of promoting outcrossing (e.g., herkogamy, dichogamy, self-incompatibility, or monoecy). To the extent that these devices prevent inbreeding, the evolution of dioecy in these species cannot logically be attributed to selection for outcrossing. In these cases, the evolution of dioecy is, we believe, due to selection for sexual specialization. However, in other species, that lack outbreeding close relatives, dioecy may have evolved from gynodioecy (males and hermaphrodites) as an outbreeding device. Subsequent disruptive selection and selection for sexual specialization may have also shaped the evolution of dioecy from gynodioecy in these species, resulting in two genetically determined, constant sex morphs.Both pathways for the evolution of dioecy require the operation of disruptive selection, though the gynodioecy route involves more restrictive disruptive selection and a genetic designation of gender. In contrast, the monoecy route is not dependent on the genetic designation of two sex morphs, but, rather, allows the possibility of sexual intermediates and sexual lability. Both pathways produce one morph in which maleness is suppressed and another in which the female function is negligible or nonexistent—the reproductive mode recognized as dioecy.Evidence is presented here to support the thesis that instances of sexual lability, the presence of an array of sexual intermediates, sex-switching, and sexual niche segregation can be explained in terms of the pathway that was taken in the evolution of a particular dioecious species. In addition, the degree of sexual dimorphism seen in dioecious species is correlated with mode of pollination (insector wind-pollinated) and other ecological factors.

Plant reproductive strategies and resource allocation.

A major theme in plant evolutionary ecology concerns the allocation of resources to fitness-enhancing activities. Patterns of allocation have been regarded as reflecting evolved reproductive strategies. Here I review recent studies that focus on problems of resource allocation. These studies concern resource currencies, plant ontogeny and bud dynamics, reproductive costs, and clonal growth. While there is a certain amount of tension between the descriptive utility and the evolutionary validity of the reproductive strategy concept, the idea of strategy remains a robust one.A major theme in plant evolutionary ecology concerns the allocation of resources to fitness-enhancing activities. Patterns of allocation have been regarded as reflecting evolved reproductive strategies. Here I review recent studies that focus on problems of resource allocation. These studies concern resource currencies, plant ontogeny and bud dynamics, reproductive costs, and clonal growth. While there is a certain amount of tension between the descriptive utility and the evolutionary validity of the reproductive strategy concept, the idea of strategy remains a robust one.

Leaf demography and clonal growth in female and male Rumex Acetosella

The cost of reproduction by female plants generally exceeds that of male plants because females produce seeds and fruits in addition to flowers (Darwin 1877, Lloyd and Webb 1977, Willson 1983). If the vegetative growth of female and male plants is equivalent, then the greater female reproductive effort should result in reduced growth and/or survival. In a dioecious clonal plant that reduction might be manifest as lower ramet production, and the sex ratio of ramets should become increasingly male-biased. In contrast to this expectation the ramet sex ratio of Rumex acetosella tends to be female-biased (Harris 1968, Putwain and Harper 1972, Lovett Doust and Lovett Doust 1985), even though females allocate a greater proportion of their resources to reproduction than do males (Putwain and Harper 1972). In eight Massachusetts populations of Rumex acetosella, we found one male-biased population and a female bias at the other sites. In these latter seven sites the estimated sex ratio of genets (the genetic individuals) did not differ significantly from 1: 1 (Lovett Doust and Lovett Doust 1985). The greater reproductive effort by female plants, plus the female bias in sex ratio of ramets suggested differential growth by females. Biased sex ratios in clonal plants can be caused by a variety of factors, including, for example, differential grazing by herbivores (see Lovett Doust and Lovett Doust 1985, Agren 1987), sex-associated effects of intraspecific competition (e.g., Onyekwelu and Harper 1979, Lovett Doust et al. 1987), and differences in the numbers of ramets produced and sustained by female and male plants (Sakai and Burris 1985). We have examined and rejected the possibility that differential herbivory on male and female Rumex plants drives the ramet sex ratio to being female-biased. (In fact, although males and females suffered comparable herbivory in absolute terms, the proportion of the available foliage that was removed from females was greater [Lovett Doust and Lovett Doust 1985].) The present study was designed to explore the hypothesis, suggested by David Lloyd (personal communication), that differences in the vegetative vigor and seasonal phonology of males and females might explain the female-biased sex ratios generally found in populations of Rumex acetosella.

American Wildcelery, Vallisneria americana, as a Biomonitor of Organic Contaminants in Aquatic Ecosystems

This paper assesses the value of American wildcelery, Vallisneria americana, as a biomonitor of organochlorine contamination. Plants provide a valuable indicator of local environmental conditions and sub-lethal effects of contaminants on growth and reproduction provide a means of assessing both acute and chronic contaminant effects. In a field survey, Vallisneria plants in the St. Clair and Detroit rivers were found to accumulate significant amounts of organochlorine contaminants, and the concentration increased over the season. Root tissue contained the highest concentrations on each occasion, suggesting contaminant transfer occurred from sediments to the roots. A factorial experiment was set up at two stations in the channel connecting Lakes Huron and Erie to assess the separate effects of contaminant concentrations in the sediment, water column, and source population of the plants, upon growth and reproduction of Vallisneria plants. Contaminant concentrations in sediment and plant tissues were also measured to correlate contaminant content and demographic effects. A clear relationship between exposure to contaminants and effect (on plant performance) was observed. Results indicated that exposure first to the water column, and secondarily to the sediments from the more contaminated site had detrimental effects on plant performance and survival. Plants at each site appeared to be locally adapted and differed in their apparent resistance to organochlorine contaminants. An “impairment index” (reflecting relative plant performance) was calculated and can be used to calibrate the degree of contamination of different sites relative to a “clean” reference site. This may allow prioritization of remediation of contaminated sites, and should allow field managers to track and document the restoration of environmental quality in recognized Areas of Concern in the Great Lakes.

Parental Strategy: Gender and Maternity in Higher Plants

Individual higher plants may be male or female to a varying degree. The division of sexual labor leaves the maternal parent in any successful mating bearing the additional cost of nurturing embryos within developing seeds. Patterns of parental sex allocation reflect the evolutionary fitness that may be gained by assuming various gender roles. The evolution of gender separation is discussed. (Accepted for publication 15 August 1982)

Labile sex expression and dynamics of gender in Thymelaea hirsuta

Gender phenotypes of Thymelaea hirsuta (L.) Endl. were recorded at five natural populations during regular visits occurring 3–6 times per year over a period of 3–6 years. Plant gender was quantifie...

Seed germination in Vallisneria americana: Effects of cold stratification, scarification, seed coat morphology and PCB concentration

Abstract:Fruits of Vallisneria americana were collected in late September from four sites along the Huron-Erie Corridor, on the Canadian side of this connecting channel between Michigan and Ontario...

Effects of trichlorethylene, plant sex and site of origin on modular demography in Vallisneria americana

Summary 1. The demographic effects of an organochlorine contaminant on the dioecious aquatic macrophyte Vallisneria americana are reported. Plants were sampled from two natural populations in the Huron-Erie corridor of the Great Lakes Basin, and placed in experimental treatments of high (396 mg per litre), medium (132 mg per litre), low (66 mg per litre) and zero concentrations of trichloroethylene (TCE). Plant responses were determined in terms of genet mortality (i.e. death of the entire genetic individual), ramet production (clonal growth) and mortality, leaf birth- and death rates, leaf area and sexual reproduction. 2. TCE in the water caused significant ramet and genet mortality, and reduced the growth of surviving plants. Control plants produced significantly more leaves than contaminated plants. Contaminant-exposed plants continued producing their leaves for 4-6 weeks longer than controls, and showed greater rates of leaf death. The total leaf area of genets exposed to TCE was significantly lower than that of unexposed plants. TCE was associated with a significant decrease in flower production. 3. The concentration of TCE was determined in plant tissues, water and sediment. Levels of TCE were greatest in the sediment, and sediment originating from each site adsorbed TCE to differing degrees. TCE was more concentrated in the underground storage tissues (roots and turions) than it was in leaves. Root and turion tissues of plants from one site contained significantly less TCE than those of plants from the other site, despite an identical concentration of TCE in the water; differences may have been due to contrasting sediment affinities for TCE, and/or different rates of TCE accumulation by the plants from each site. 4. TCE was bioaccumulated, particularly in roots. This appeared to be attributable to the fact that TCE accumulated, in the first instance, in sediments. The logIo of TCE concentrations in sediment were linearly correlated with TCE concentrations in roots, suggesting that TCE uptake is through the root system rather than the leaves, and that uptake is driven by the concentration gradient between sediment and plant. 5. Plants that survived the TCE treatment appeared to be resistant to the chemical; they were as likely to flower as surviving untreated plants. Males were more likely than females to survive TCE treatment. 6. It is concluded that modular demography can be used to characterize the performance of plants that are stressed by exposure to various concentrations of the organochlorine solvent trichloroethylene, and that plants from different sites differ in their ability to survive contaminant stress.