Small-scale and regional spatial dynamics of an annual plant with contrasting sexual systems

Plant demography is known to depend on both spatial dynamics and life history, but how these two factors interact is poorly understood. We conducted a longitudinal study of the wind-pollinated annual plant Mercurialis annua that varies geographically in its sexual system to investigate this interaction. Metapopulation demographic models predict that regular population turnover should be a more common feature of monomorphic than dimorphic populations because males and females cannot found new populations by selfing but hermaphrodites can. We tested the prediction that rates of population turnover would be higher in monomorphic compared to dimorphic regions. We surveyed 356 populations of M. annua along five regional transects in Morocco and the Iberian Peninsula over a 3-year period to examine their demography and persistence. Each transect crossed a transition in the sexual system, from a monomorphic region where almost all populations were hermaphroditic to a dimorphic one in which most populations had separate sexes (males with females or hermaphrodites). As predicted, rates of local apparent extinctions (i.e., the disappearance of adult plants) were nearly 50% higher in monomorphic compared to dimorphic regions. Local extinctions appeared to be driven by changes in vegetation cover, with extinctions tending to occur in sites in which perennial cover also declined. This suggests that disturbance is a primary agent of local extinctions. We further examined the influence of regional dynamics on local demographic properties by investigating patterns of spatial autocorrelation in population density across years. We found positive spatial autocorrelations in plant densities within regions for both sexual systems. However, these positive autocorrelations extended over shorter distances in monomorphic regions, perhaps as a result of greater population flux in these regions. Synthesis. Our study shows that population dynamics may be influenced by processes acting at a range of spatial scales: within patches, across patches within sites, and across sites within regions, as well as by life-history variation. In Mercurialis annua, regional variation in apparent extinction rates is affected by life history and implicated in regulating the geographical distribution of populations with different sexual systems

Two's Company, Three's a Crowd: Experimental Evaluation of the Evolutionary Maintenance of Trioecy in Mercurialis annua (Euphorbiaceae)

Trioecy is an uncommon sexual system in which males, females, and hermaphrodites co-occur as three clearly different gender classes. The evolutionary stability of trioecy is unclear, but would depend on factors such as hermaphroditic sex allocation and rates of outcrossing vs. selfing. Here, trioecious populations of Mercurialis annua are described for the first time. We examined the frequencies of females, males and hermaphrodites across ten natural populations and evaluated the association between the frequency of females and plant densities. Previous studies have shown that selfing rates in this species are density-dependent and are reduced in the presence of males, which produce substantially more pollen than hermaphrodites. Accordingly, we examined the evolutionary stability of trioecy using an experiment in which we (a) indirectly manipulated selfing rates by altering plant densities and the frequency of males in a fully factorial manner across 20 experimental plots and (b) examined the effect of these manipulations on the frequency of the three sex phenotypes in the next generation of plants. In the parental generation, we measured the seed and pollen allocations of hermaphrodites and compared them with allocations by unisexual plants. In natural populations, females occurred at higher frequencies in denser patches, a finding consistent with our expectations. Under our experimental conditions, however, no combination of plant densities and male frequencies was associated with increased frequencies of females. Our results suggest that the factors that regulate female frequencies in trioecious populations of M. annua are independent of those regulating male frequencies (density), and that the stable co-existence of all three sex phenotypes within populations is unlikely

Sex ratio variation in gynodioecious species of Echium endemic to the Canary Islands

Abstract: Species of Echium from the Canary Islands represent an adaptive radiation of plants involving evolutionary changes in habitat and life form. Alongside these changes, evolutionary transitions of the sexual system have occurred: approximately one quarter of Echium species from the Canary Islands are dimorphic for gender, paralleling other adaptive radiations of plants on oceanic islands, such as Schiedea in Hawaii. Here, I report on the variation in the frequency of female plants among eight gynodioecious populations representing four species of Echium on the island of Tenerife, and provide preliminary estimates of the seed fertility of females and hermaphrodites, which is an important determinant of the sex ratio for gynodioecious species. Across populations, female frequencies ranged between 6% and 54%, a much wider range than had previously been reported for gynodioecious species of Echium on the Canary Islands. Substantial frequencies of female plants occurred, even though no differences in the seed fertility of females and hermaphrodites were detected. Key words: Canary Islands, Echium, island radiation, female frequency, gynodioecy, sex ratio. Résumé : Les espèces d'Echium des îles Canaries représentent une radiation adaptative de plantes impliquant des changements évolutifs dans l'occupation de l'habitat et dans la forme biologique. En plus de ces modifications, on observe des transitions évolutives du système sexuel; environ le quart des Echium des îles Canaries sont dimorphes quant au genre, ce qui correspond à la radiation des espèces sur d'autres îles, comme les Schiedea en Hawaii. L'auteur rapporte ici la variation des fréquences femelles au sein de huit populations gynodioïques représentant quatre espèces sur l'île de Ténériffe, et fournissant les mesures préliminaires de la fertilité des plantes femelles et hermaphrodites par leur fonction femelle, un important facteur déterminant pour le rapport des sexes chez les espèces gynodioïques. La fréquence des femelles varie entre 6 % et 54 %, un écart beaucoup plus large que ce qui a déjà été rapporté pour les Echium des îles Canaries. On observe une fréquence femelle substantielle, même si aucune différence n'a pu être observée dans la fertilité des graines des plantes femelles et hermaphrodites

MomType

These data were used to conduct an analysis of deviance between the identity of the father (male vs. hermaphrodite) and identity of the mother (female vs. hermaphrodite) using a contingency table approach. The column labels are: Treatment (levels of the nutrient resource addition treatment - High vs. Low); Father (the sex of the father that sired the seeds genotyped in the paternity analysis - M for male, H for hermaphrodite); Mother (the sex of the mother that produced the seeds genotyped for paternity analysis); Count (the total number of seeds screened for each treatment and sex combination)

FieldData

Data collected from natural populations of S. latifolia, as described in the paper. Column names refer to the following information: Site (sampling location); SS (sexual system - monoecious or dioecious); Mopen (number of open male flowers per inflorescence); Mtotal (total number of male flowers per inflorescence); MVL (leaf mid-vein length for the leaf subtending the inflorescence, cm); Petal (petal width, cm)

Data from: The evolution of males: support for predictions from sex allocation theory using mating arrays of Sagittaria latifolia (Alismataceae)

Investment in male function should often yield diminishing fitness returns, subjecting the evolution of male phenotypes to substantial constraints. In plants, the subdivision of male function via the gradual presentation of pollen might minimize these constraints by preventing the saturation of receptive stigmas. Here we report on an investigation of (a) patterns of investment in male function by plants in hermaphroditic (monoecious) and dioecious populations of Sagittaria latifolia, and (b) patterns of siring success by males vs. hermaphrodites in experimental mating arrays. We show that in natural populations, males from dioecious populations had greater investment in male function than hermaphrodites in monoecious populations. However, as a proportion of total flower production, males presented substantially fewer flowers at once than hermaphrodites. In comparison with hermaphrodites, therefore, males prolonged the period over which they presented pollen. In mating arrays comprised of females, males, and hermaphrodites, siring success by males increased linearly with flower production. This finding is consistent with the existence of a linear gain curve for male function in S. latifolia and supports the idea that the gradual deployment of male function enables plants to avoid diminishing returns on the investment in male function

ArrayDataPlantFlowerSize

Plant size and flowering data for the plants used in the mating arrays. Column labels refer to: Array (the array number; nb. these do not correspond with the labels in Fig. 2, see data file description for the cumulative siring data set for a key); Treatment (high vs. low nutrient resource additions); Plant (code name for the genotype included in the array); Sex (H - hermaphrodite, M - male); MVL (leaf mid vein length for the leaf subtending the inflorescence); Nflowers (total number of flowers produced per inflorescence); Year (year the genotype had been sampled from natural populations)

SiringCountData

These data were used to evaluate overall differences in the number of seeds sired by males vs. hermaphrodites. Column labels are: Array (label for the mating array); Treatment (level of the nutrient resource addition treatment - High vs. Low); SexofSire (plant sex (male or hermaphrodite) to correspond with the data in the next column; Progeny (the number of seeds sired by the focal male, or the average number of seeds sired by the hermaphrodites in each array); Fflowers (the total number of female flowers available for siring opportunities in each array)

Cumulative_floweringBYsiring

The data set contains the data used to generate Figure 1, and to analyse the association between flower production and siring success. Column names are as follows: Treatment (levels of the nutrient resource addition treatment - High and Low); Array (label for the mating array); CumFlws (cumulative number of male flowers produced by the focal male per day for the duration the array was set up); CumSire (cumulative number of seeds sired by the focal male over the course of the array)

PropSiring

These data correspond with those presented in Fig. 2. Column labels: Array (label for array); Trt (level of the nutrient resource addition treatment - High vs. Low); Mflws (the total number of flowers produced by the focal male in the array); Hflws (the total number of hermaphrodite flowers produced by all hermaphrodites per array); Totflws (the sum of Mflws and Hflws); Mprog(the total number of seeds sired by the focal male per array); TotProg (the total number of seeds genotyped per array)