Variation in sexual reproduction in orchids and its evolutionary consequences: a spasmodic journey to diversification

The great taxonomic diversity of the Orchidaceae is often attributed to adaptive radiation for specific pollinators driven by selection for outcrossing. However, when one looks beyond the product to the process, the evidence for selection is less than overwhelming. We explore this problem by discussing relevant aspects of orchid biology and asking which aspects of reproduction explain the intricate pollination mechanisms and diversification of this family. We reaffirm that orchids are primarily pollination limited, the severity of which is affected by resource constraints. Fruit set is higher in temperate than in tropical species, and in species which offer pollinator rewards than those that do not. Reproductive success is skewed towards few individuals in a population and effective population sizes are often small. Population structure, reproductive success and gene flow among populations suggest that in many situations genetic drift may be as important as selection in fostering genetic and morphological variation in this family. Although there is some evidence for a gradualist model of evolutionary change, we believe that the great diversity in this family is largely a consequence of sequential and rapid interplay between drift and natural selection.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 84 , 1–54.     

FEEDING AND GROWTH OF THE AEOLID NUDIBRANCH CUTHONA NANA (ALDER AND HANCOCK, 1842)

Feeding and growth of the aeolid nudibranch Cuthona nana wasdocumented. Cuthona nana feeds specifically on the colonialhydroid Hydractinia echinata. Nudi-branchs were observed onhermit crab shells in Gos-port Harbor at the Isles of Shoals,Maine and on old bridge pilings near Gerrish Island, Maine. Grazing rates of nudibranchs (3–25 mm) were quantifiedover 24 hour periods in the laboratory. Larger nudibranchs consumedmore hydroid tissue feeding mostly on polyps, whereas smallerindividuals consumed less and fed on mat tissue. Grazing intensityin the field at Gosport Harbor was determined by estimatingthe area of grazed patches of hydroid colonies attached to hermitcrab shells. The average patch size was 5.75% of the total colonyarea. Growth rates for nudibranchs were determined in the laboratory.Growth rates were greatest prior to maturation and oviposition.Animals from Gerrish Island and Gosport Harbor decreased insize after oviposition due to senescence. Nudibranchs from GerrishIsland reached a maximum size of 12 mm, compared to 23 mm foranimals at Gosport Harbor. ^*Present address: Division of Science and Mathematics. Universityof Minnesota, Morris. Minnesota 56267, USA     

Hybridization and sex pheromone responses in two closely related mealybug species (Homoptera: Pseudococcidae)

Abstract. Reciprocal crosses were made between the closely related species Planococcus citri and Planococcus ficus by using mating inducers. The pheromone responses of the hybrid offspring were bioassayed. The sex ratio of the offspring showed a striking shift in favour of females in comparison with homospecific matings mainly because of high mortality of male nymphs during the first instar. Hybrid males gave positive reactions only to pheromone extracts from the maternal species and showed no response to extracts from the paternal species. This result may be explained by the lecanoid chromosome system known for mealybug males, in which the paternal set of chromosomes becomes heterochro-matic (condensed and genetically inactive) starting from early embryogeny. Further evidence is, however, presented that (at least in certain tissues) the paternal set reverts to the euchromatic state and to genetical activity.     

Gene flow and effective population size in Lepanthes (Orchidaceae): a case for genetic drift

Genetic drift can play an important role in population differentiation, particularly when effective population sizes are small and gene flow is limited. Such conditions are suspected to be common in the species-rich Orchidaceae. We investigated the likelihood of genetic drift in natural populations of three endemic species of Lepanthes (Orchidaceae) from Puerto Rico. We estimated effective population size, Ne, using three ecologically based methods. Two of the three estimates were based on variance in reproductive potential and the third was based on coalescence time. All estimates of Ne were usually <40% of the standing population size, resulting in values of <20 individuals per population. Based on starch gel electrophoresis of isozymes, Nm estimates suggest restricted gene flow among populations in the range of one or less successful migrant per generation. Genetic differentiation among populations is expected under such conditions from random genetic drift. Indeed we observed high genetic differentiation among populations (L. rubripetala, F_ST> G_ST, 6; θ.248, 0.266, 0.293; L. rupestris, 0.148, 0.169, 0.138; L. eltoroensis, 0.251, 0.219, 0.218, respectively). Genetic drift is likely to be important for population differentiation in Lepanthes as a result of small effective population sizes and restricted gene flow.     

Morphological flexibility across an environmental gradient in the epiphytic orchid,Tolumnia variegata: complicating patterns of fitness

Deceit‐pollinated orchid species show substantial variation in floral traits, which may be maintained by genetic drift or various forms of selection, or may reflect phenotypic plasticity. We explored how much plasticity occurs in both vegetative and floral traits of Tolumnia variegata (Oncidiinae, Orchidaceae) across two different light environments in Puerto Rico using data from a reciprocal transplant experiment. We also examined how fruit set, a measure of reproductive success and a surrogate for fitness, is associated with this morphological variation, and whether it changes over time. Tolumnia variegata responded to environmental variables in multiple ways. Vegetative characters were more plastic than those associated with sexual reproduction. Transplant effects accounted for significant variation in flower length, lip length, number of inflorescences, peduncle length, leaf length and the total number of ramets, but responses were not always consistent among years. Phenotypic selection on morphological characters was dependent on plant location. The trends detected were complex, and often inconsistent across years, probably as a result of wetter and drier years than average. Overall fruit set was quite variable among plants, averaging 15%, with no significant differences among sun and shade plants. Although reproductive success was similar among sites, habitat heterogeneity and annual variation had an effect on morphological expression, which sometimes modified the trajectories of phenotypic selection. © 2010 The Linnean Society of London, Botanical Journal of the Linnean Society, 2010, 163 , 431–446.     

The effect of flower position on male and female reproductive success in a deceptively pollinated tropical orchid

In many plants, including orchids, differential fruit set along the inflorescence has been attributed to pollinator behaviour. For instance, the pollinator, moving up the inflorescence, becomes satiated with the resources and leaves before visiting the upper flowers. Consequently, the pollinators do not visit flowers as frequently higher up the inflorescence. Alternatively, flower size may vary along the inflorescence, making pollination ineffective as flowers decrease in size. I tested for the presence of differential pollination along the inflorescence in a pollinator-limited tropical epiphyte, Lepanthes rupestris Stimson, and determined the likely cause of the observed pattern. As this species has inflorescences with sequential flowering, pollinator behaviour, moving up the inflorescence as in synchronous multiflowering inflorescences, can be discounted as an explanation for differential fruit set. Fruit set is shown to be more frequent at the base of the inflorescence, but male reproductive success through pollinarium removal is basically independent of flower position. Moreover, cross-pollination by hand at variable flower positions along the inflorescence results in equal fruit set, suggesting that resources are not limiting and cannot explain the cause of differential fruit production along the inflorescence in natural populations. Furthermore, flower size is shown to diminish along the inflorescence, suggesting that the pollinator(s) may be ineffective at depositing the pollinarium in the smaller higher flowers. Consequently, pollinator behaviour and its interaction with flower size, and not resource limitation, is likely to be the main cause of differential fruit set along the inflorescence in L. rupestris .  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 405–410.     

Observations with Hoechst Staining of Amitosis in Acanthamoeba castellanii

DNA staining with Hoechst 33258 allows us to visualize and to characterize amitotic division of Acanthamoeba castellanii in which the cell nucleus divides without DNA condensation and regular segregation of chromosomes. Amitosis involves several programmed and coordinated steps that exclude accidental cutting of the cell nucleus.