John D. Thompson

Phenotypic plasticity as a component of evolutionary change.

Phenotypic plasticity has often been assumed to buffer the effects of natural selection and thus act as a constraint on evolutionary change. It has become increasingly clear, however, that phenotypic plasticity actually represents a fundamental component of evolutionary change. Where genetic variation for plasticity exists, a population with a different mean plasticity can evolve. Recent attention has been focused on the conditions necessary for the evolution of phenotypic plasticity, i.e. those under which a generalist strategy, as opposed to a range of genetically differentiated specialists, will be favoured. It is also now clear that genotypes that perform best in one environment usually perform less well than other genotypes in a different environment; hence, their greater response is not an adaptation to environmental variation. A response to environmental variation is only adaptive if it represents a mechanism by which relative fitness is maintained in the face of environmental variation. Adaptive plasticity may thus involve both physiological homeostasis and morphological response.

QUALITATIVE AND QUANTITATIVE VARIATION IN MONOTERPENE CO-OCCURRENCE AND COMPOSITION IN THE ESSENTIAL OIL OF THYMUS VULGARIS CHEMOTYPES

Thymus vulgaris has a chemical polymorphism with six different chemotypes that show marked spatial segregation in nature. Although some populations have a single chemotype in majority, many have two or three chemotypes. In this study we analyze the quantitative variation among T. vulgaris populations in the percentage of oil composed of the dominant monoterpene(s) for each chemotype. In general, phenolic chemotypes (thymol and carvacrol), which occur at the end of the biosynthetic chain, have a significantly lower proportion of their oil composed of their dominant monoterpene than nonphenolic chemotypes (geraniol, α-terpineol, and linalool). This is due to the presence of high amounts of precursors (γ-terpinene and paracymene) in the oil of phenolic chemotypes. The essential oil of the nonphenolic thuyanol chemotype has four characteristic monoterpenes that together make up a lower proportion of the oil than the single dominant monoterpene of the other nonphenolic chemotypes. For all chemotypes, the percentage composition of the dominant monoterpene decreased significantly at sites where the chemotype is not the majority type. This decrease is correlated with a significant increase in either the proportion of the two precursors for the thymol chemotype or the monoterpenes characteristic of the other chemotypes at the site. The latter result suggests that a plant with dominant genes is responsible for the production of different monoterpenes can produce several molecules.

Population differentiation in Mediterranean plants: insights into colonization history and the evolution and conservation of endemic species

Colonization and isolation are critical events in the evolutionary dynamics of plant populations. In this paper I review how spatial population structure of genetic markers provides insights into the evolutionary significance of episodes of colonization and isolation in the Mediterranean flora. I use as themes to structure my review the following topics: spatial structure induced by historical associations among populations of widespread species; population differentiation in relation to the evolution of closely related species with disjunct distributions; the potential effect of founder events during colonization on character evolution; and the conservation implications of spatial population structure. My review illustrates that the Mediterranean flora is full of examples that provide key insights into such evolutionary and conservation issues.

Distyly and variation in heteromorphic incompatibility in Gaertnera vaginata (Rubiaceae) endemic to La Reunion Island

Documenting the floral biology of species throughout the Rubiaceae family is of particular interest since heterostyly and dioecy may have evolved more than once in this large family. Unfortunately many species in several tropical regions remain unstudied. The purpose of this paper is to describe the floral biology, the nature of self-incompatibility, morph ratios, and fecundity in natural populations of Gaertnera vaginata, a small tree endemic to the island of La Reunion in the Indian Ocean. Measurements of floral characters in populations across the entire distribution of this species showed that G. vaginata exhibits a reciprocal stigma height and anther height dimorphism characteristic of a distylous species. Pollen grain size and corolla tube length are consistently greater in short-styled plants and long-styled plants produce more pollen per flower. Controlled pollinations in a natural population showed that 25% of the short-styled plants gave at least one fruit on intramorph (illegitimate) pollination, whereas no long-styled plants set fruit on illegitimate pollination. In total, 19.4% of illegitimate pollinations produced fruit on short-styled plants. No self-pollination gave fruit on either morph and between-morph pollinations produced 92.2 and 92.8% for short and long-styled plants, respectively. Overall, short-styled plants were significantly more abundant than long-styled plants. Short-styled plants outnumbered long-styled plants in 16 of the 19 populations. In three of these populations the morph ratio was significantly different from 1:1. In two natural populations, fruit set was significantly higher on long-styled plants, although the number of seeds per fruit was not significantly different between the two morphs. The possible effect of variation in the strength of heteromorphic incompatibility on observed variation in morph abundance and the possible causes for the variation in fruit set are discussed.

Variation of pollen limitation in the early flowering Mediterranean geophyte Narcissus assoanus (Amaryllidaceae)

Abstract A recent literature review indicates that pollen limitation of female fertility is a common feature of flowering plants. Despite the ecological and evolutionary significance of pollen limitation, most studies have only examined fertility in a single population at one time. Here we investigate pollen limitation of fruit and seed set in five populations of Narcissus assoanus, a self- sterile, insect-pollinated geophyte, over 2–3 years in southern France. In common with many early spring flowering plants, pollinator visitation to N. assoanus is often infrequent. Supplemental hand-pollination of flowers with outcross pollen significantly increased overall fruit and seed set by 11% and 19%, respectively. Four of the five populations experienced some pollen limitation during the study. For a given year, there was significant variation in pollen limitation among populations. Two of the populations were pollen limited in one year but not in other years in which they were studied. Seed:ovule ratios for open- and hand-pollinated flowers averaged 0.29 and 0.33, respectively. While hand pollination significantly increased the seed:ovule ratio, the low value obtained indicates that the majority of ovules in flowers do not mature seeds despite hand pollination. The role of genetic and environmental factors governing low seed:ovule ratios in N. assoanus is discussed.

Species diversity and ecological range in relation to ploidy level in the flora of the Pyrenees

Polyploidy is a major process in plant evolution. Surprisingly, no study has examined its role in species diversification and ecological distribution in relation to other life history traits. In this study, we examine to what extent polyploidy and the other traditionally examined biological traits (pollination mode, dispersal mode and growth form) account for ecological and taxonomic diversity in the flora of the Pyrenees. Fifty genera (in 22 angiosperm families) were classified according to ploidy level, growth form, pollination mode and dispersal mode, and 451 species and/or subspecies in these 50 genera were classified according to ploidy level and growth form. We examined the contribution of ploidy level, pollination and dispersal modes and growth form to (i) the ecological range of species and genera, i.e., the number of natural habitats (defined by a combination of ecological characteristics) where they occur, and (ii) the taxonomic diversity of the 50 genera. Ploidy level and dispersal mode had significant effects on the taxonomic diversity of the 50 genera. Taxonomic diversity, but not polyploidy per se, was significantly correlated with ecological range of genera. For individual species, diploids had a larger ecological range than polyploids, and herbaceous growth forms had wider ecological distributions than other growth forms. Our results indicate that polyploidisation may be a source of ecological diversification of genera, not by increasing the ecological range of particular polyploid species compared to diploids, but rather by creating taxonomic diversity that leads in some genera to a diversification of the habitats occupied by different ploidy levels. This observation is consistent with previous observations of ecological divergence of chromosomal races in some species in the Alps and in the Pyrenees. As found in other studies, species diversification in the studied flora appears to be greatly influenced by the occurrence of multiple dispersal modes, while ecological range of species or subspecies is significantly increased by the presence of herbaceous species.

The rise of research on futures in ecology: rebalancing scenarios and predictions.

Concern about the ecological consequences of global change has increasingly stimulated ecologists to examine the futures of ecological systems. Studying futures is not only a crucial element of the interaction between science, management and decision making, but also a critical research challenge per se, especially because futures cannot be observed or experimented on. In addition, researchers can encounter methodological and theoretical difficulties, which make interpretations and predictions problematic. In the literature which deals with futures of ecological systems two main lines of research can be distinguished: a predictive approach, which dominates the literature, can be contrasted with a rarer number of studies that elaborate potential scenarios for ecological systems. Scenario approaches currently concern mainly contacts with stakeholders or decision makers, or the use of climate scenarios to derive projections about ecological futures. We argue that a new direction for ecological futures research could be explored by using ecological scenarios in combination with predictive models to further fundamental ecological research, in addition to enhancing its applied value.

The effects of population size limitation on fecundity in mosaic populations of the clonal macrophyte Scirpus maritimus (Cyperaceae).

The clonal macrophyte Scirpus maritimus (Cyperaceae) propagates locally by rhizomes and reproduces sexually by achenes. The purpose of this paper was to examine whether in size-limited habitats in patchy and discrete marshes in two Mediterranean wetlands in southern France natural populations may suffer from a reduced maternal fecundity due to a deficit in outcross pollen. We first verified that S. maritimus suffers from a reduced fecundity when self-pollinated. At a site in the Camargue, mean fecundity (mean number of achenes per centimetre of spikelet) measured in 1995 and 1996 in seven and nine populations, respectively (surface area from 50 to 4500 m) increased significantly with population surface area in 1995 but not in 1996. In the second wetland at Roquehaute, which is composed of small ponds, fecundity was very low in all 12 local populations studied in 1996 (1.1 achenes per spikelet, SD = 1.2) and was not correlated with the population surface area (from 10 to 400 m). We performed a pollen supplementation experiment in five local populations at Roquehaute to determine whether this low fecundity may be due to a pollen limitation. A significant increase in fecundity after among-pond pollinations compared to within-pond pollinations indicated that local populations suffer from a deficit in outcross pollen, since each pond appears to contain one or a few number of clones (or incompatibility types). In S. maritimus, clonal spread may have a cost in terms of reduced fecundity in small habitats because each habitat is colonized by very few clones.

Variation in phenotypic response to light availability between diploid and tetraploid populations of the perennial grass Arrhenatherum elatius from open and woodland sites

1 Phenotypic plasticity and genetic differentiation may influence how natural populations colonize variable environments, and thus help explain differences in distribution of closely related taxa. The response to shading in woodland and open habitat populations of endemic diploid and widespread tetraploid Arrhenatherum elatius was studied. 2 Two questions were examined. (i) Does the widespread tetraploid A. elatius ssp. elatius show greater phenotypic homeostasis in fitness-related traits than the related Mediterranean endemic diploid ssp. sardoum? (ii) Do the two ploidy levels differ in patterns of population differentiation related to the original habitat? 3 For each ploidy level, 10 genotypes from each of three populations from woodland and three from open habitats were grown in a garden environment at two light intensity levels. 4 No differences in phenotypic plasticity of morphological, phenological and fitnessrelated traits studied were observed between the ploidy levels. Tetraploids showed greater vegetative and inflorescence size. 5 Tetraploid populations showed evidence of differentiation in relation to original habitat conditions: plants from open habitats had higher seed production, were taller and flowered earlier than populations from woodland habitats in both garden environments, but phenotypic plasticity did not differ between the two habitat types. No variation in phenotypic plasticity or population differentiation was observed among diploid populations from either habitat. 6 The lack of differences in plasticity between diploids and tetraploids, and the greater differentiation between woodland and open populations in tetraploids, are discussed in relation to the geographical distribution of the two ploidy levels.

Genetic structure of continental and island populations of the Mediterranean endemic Cyclamen balearicum (Primulaceae).

Cyclamen balearicum is a self-compatible perennial herb endemic to the western Mediterranean Basin. This species occurs in five geographically isolated terrestrial islands in southern France and on four Balearic islands. In this study, we compare genetic variability and differentiation within and among 11 terrestrial island populations and 17 true island populations. Of nine readable enzyme loci, five were polymorphic in both terrestrial and true islands. F statistics showed a significant heterozygote deficiency in all populations, probably due to high levels of autonomous selfing, restricted gene flow, and subsequent genetic drift. Genetic diversity was higher in terrestrial islands than on the Balearic islands, suggesting that the Balearic islands were colonized when they were in contact with the continent. Population differentiation was greater among terrestrial islands (Fst = 0.417 and Gst = 0.344) than among true islands (Fst = 0.112 and Gst = 0.093). Furthermore, differentiation among populations on the Basses Cévennes terrestrial island was greater (Fst = 0.254) than among populations on the true island of Mallorca (Fst = 0.163). The greater genetic differentiation among terrestrial islands could have been caused by genetic bottlenecks associated with changes in climate and human land use that may have reduced population sizes more severely in terrestrial islands in southern France than on the Balearic islands.