The effect of timing of pollination on the mating system and fitness ofKalmia latifolia (Ericaceae)

Plant mating systems are known to vary within species and some immediate ecological factors have been found to be associated with the geographic distribution of selfing. The environmental condition of the maternal plant may influence the production of selfed seed relative to outcrossed seed. This study investigated the effect of late pollination on the mating system of Kalmia latifolia, a long-lived perennial shrub. A 2 × 2 experimental design was used to determine whether reproductive success declines over the course of the flowering season and whether there was an interaction between pollination time (early vs. late in the season) and pollen type (self-fertilized vs. outcrossed). An interaction of this sort would indicate context-dependent fitness of selfed seeds compared to outcrossed seeds and, thus, show an ecological influence over a plant's mating system. Relative fitness was assessed in terms of female reproductive success. Timing of pollination did not affect abortion of outcrossed seeds; however, delay in pollination increased abortion of selfed seeds by 34.7%. Thus, it appears that plants selectively aborted selfed seeds rather than outcrossed seeds and this selection was more intense at the end of the season. An ecological factor such as time of pollination may affect the mating system of K. latifolia.     

Phenotypic Plasticity of the Introduced New Zealand Mud Snail,Potamopyrgus antipodarum,Compared to Sympatric Native Snails

Phenotypic plasticity is likely to be important in determining the invasive potential of a species, especially if invasive species show greater plasticity or tolerance compared to sympatric native species. Here in two separate experiments we compare reaction norms in response to two environmental variables of two clones of the New Zealand mud snail, Potamopyrgus antipodarum, isolated from the United States, (one invasive and one not yet invasive) with those of two species of native snails that are sympatric with the invader, Fossaria bulimoides group and Physella gyrina group. We placed juvenile snails in environments with high and low conductivity (300 and 800 mS) in one experiment, and raised them at two different temperatures (16°C and 22°C) in a second experiment. Growth rate and mortality were measured over the course of 8 weeks. Mortality rates were higher in the native snails compared to P. antipodarum across all treatments, and variation in conductivity influenced mortality. In both experiments, reaction norms did not vary significantly between species. There was little evidence that the success of the introduced species is a result of greater phenotypic plasticity to these variables compared to the sympatric native species.     

Genetic constraints on floral evolution in a sexually dimorphic plant revealed by artificial selection

Sexual dimorphism is one of the most widespread and recognizable patterns of phenotypic variation in the biotic world. Sexual dimorphism in floral display is striking in the dioecious plant Silene latifolia, with males making many, small flowers compared to females. We investigated this dimorphism via artificial selection on two populations to determine whether genetic variation exists within populations for flower size and the extent of the between-sex correlation, whether a flower size and number trade-off exists within each sex, and whether pollen and ovule production vary with flower size. We selected for decreased flower size (calyx width) in females and increased flower size in males and measured the response to selection in size and correlated responses in flower dry mass, flower number, and pollen or ovule number per flower. Four bouts of selection in each of two selection programs were performed, for a total of three selection lines to decrease size, three to increase it, and two control lines. Flower size always significantly responded to selection and we always found a significant correlated response in the sex not under selection. Selection decreased but did not eliminate the sexual dimorphism in flower dry mass and number. A negative relationship between flower size and number within each sex was revealed. Whereas ovule number showed a significant correlated response to selection on flower size, pollen number did not. Our results indicate that although substantial additive genetic variation for flower size exists, the high between-sex genetic correlation would likely constrain flower size from becoming more sexually dimorphic. Furthermore, floral display within each sex is constrained by a flower size and number trade-off. Given this trade-off and lack of variation in pollen production with flower size, we suggest that sexual dimorphism evolved via sexual selection to increase flower number in males but not females.     

Genetic correlations with floral display lead to sexual dimorphism in the cost of reproduction

In dioecious plants, females typically invest more biomass in reproduction than males and consequently experience stronger life-history trade-offs. Sexual dimorphism in life history runs counter to this pattern in Silene latifolia: females acquire less carbon and invest more biomass in reproduction, but males pay a higher cost of reproduction. The species is sexually dimorphic for many traits, especially flower number, with males producing many, small flowers compared to females. We tested whether the cost of reproduction is higher in males because flower number, which we presume to be under sexual selection in males, is genetically correlated with traits that would affect life-history trade-offs. We performed artificial selection to reduce the sexual dimorphism in flower size and looked at correlated responses in ecophysiological traits. We found significant correlated responses in total vegetative mass, leaf mass, leaf thickness, and measures of CO(2) exchange. Individuals in the many-and-small-flowered selection lines did not grow as large or invest as much biomass in leaves, and their leaves exhibited an up-regulated physiology that shortened leaf life span. Our results are consistent with the hypothesis that genetic correlations between floral display and ecophysiological traits lead to a higher cost of reproduction for males.     

Parasite-induced change in host behavior of a freshwater snail: parasitic manipulation or byproduct of infection?

Host behavioral changes due to parasitism are often assumedto be adaptations of the parasite. However, behavioral effectsof parasites may be a generalized response to parasitism andonly coincidentally beneficial for parasite transmission. Forthis reason, alternatives to the manipulation hypothesis shouldbe tested. Previous work demonstrated that the trematode parasiteMicrophallus sp. influences the behavior of the snail Potamopyrgusantipodarum in a way that may increase the probability of transmission.Here I report work conducted to test alternatives to the manipulationhypothesis. In a field study, the effect of Microphallus onbehavior was compared to that of two other castrating parasitegroups to determine if the behavioral change is simply a byproductof parasitism. Also, the foraging behaviors of infected anduninfected snails were examined in the presence and absenceof food resources to determine if the hunger level of Microphallus-infectedsnails could account for the parasite-induced behavioral change.First, Microphallus-infected snails were found on top of rocksduring the day less often than the two other parasite groups. Thisevidence suggests that the behavioral change caused by Microphallusis specific to Microphallus-infected snails. Second, Microphallus-infectedsnails responded to the lack of food differently from uninfectedsnails. Uninfected snails retreated to safer positions underrocks when the food source was removed from the top of the rocks,while Microphallus-infected snails remained on top of the rockswhere the risk of consumption by the final host is greater.Taken together with previous studies, these results suggestthat infection by Microphallus results in behavior that enhancesparasite transmission.     

A Measure of the Various Modes of Inbreeding in Kalmia latifolia

Inbreeding can occur in several ways. For a given species, analysingoverall inbreeding down to its component parts can be usefulin explaining how and why inbreeding is maintained in a population.Kalmia latifolia, a long-lived evergreen shrub, was chosen forthis study for two reasons: its unique floral morphology suggestedthat it was specially adapted to assure reproduction in thedelayed mode of self-fertilization, and previous studies hadsuggested that populations that were more limited by pollinatorsshowed higher rates of within-flower self-fertilization whenpollinators were excluded. Floral manipulations, controlledpollinations, and paternity analysis were performed to determinethe most common form of inbreeding within a flower, among flowersof one individual, or among related individuals. Although plantsare capable of setting seed within a flower, the overall rateof inbreeding was low. Among the modes of inbreeding withinone individual, a negligible selfing rate among emasculatedflowers suggests that selfing is more prevalent within a flowerthan among flowers on the same plant. In addition, as the multi-locusestimate of inbreeding was less than the single-locus estimate,inbreeding among related individuals also contributes to thenatural level of inbreeding. Copyright 2000 Annals of BotanyCompany Kalmia latifolia; mountain laurel; pollination; geitonogamy; autogamy; self-fertilization; inbreeding     

Parasite-induced alteration of diurnal rhythms in a freshwater snail

The trematode Microphallus sp. alters the behavior of its snail intermediate host, Potamopyrgus antipodarum, in ways that seem to increase transmission to its final host, e.g., waterfowl, and decrease the probability of being eaten by other predators, e.g., fish. The parasite seems to cause the snail to move from the top to the bottom of rocks at about 0900 hr. Waterfowl feed predominantly before 0900 hr, and fish feed predominantly after 0900 hr. In the present study, we tested the hypothesis that Microphallus sp.-infected snails exhibit a change in behavior at around 0900 hr by examining their response to light and vertical orientation before and after 0900 hr. Results demonstrated that uninfected snails generally move toward light, oriented downward, and move a greater distance in the light compared with the dark at all times of day. Microphallus sp.-infected snails behaved differently from uninfected snails in the early morning but similarly to uninfected snails in the late morning with regard to downward orientation and distance moved in response to light. Snails infected with parasites other than Microphallus sp. behaved similarly to uninfected snails during both time periods. These results suggest that Microphallus sp. manipulates the behavior of Potamopyrgus sp. by altering rates of movement in response to light and vertical orientation in a manner consistent with the hypothesized 0900-hr shift.