Impacts of gene flow and logging history on the local genetic structure of a scattered tree species, Sorbus torminalis L. Crantz

Sorbus torminalis L. Crantz is a colonizing tree species usually found at low density in managed European forests. Using six microsatellite markers, we investigated spatial and temporal patterns of genetic structure within a 472-ha population of 185 individuals to infer processes shaping the distribution of genetic diversity. Only eight young stems were found to be the result of vegetative reproduction. Despite high levels of gene flow (standard deviation of gene dispersal = 360 m), marked patterns of isolation by distance were detected, associated with an aggregated distribution of individuals in approximately 100-m patches. This spatial structure of both genes and individuals is likely to result from patterns of seedling recruitment combined with low tree density. Our results suggest that landscape factors and logging cycles markedly shape the distribution of favourable sites for seedling establishment, which are then colonized by sibling cohorts as a result of joint seed transportation by frugivores. These combined genetic and demographic processes result in similar genetic structure both within and among logging units. However, conversion to high forest may enhance genetic structuring.     

A Model for Antagonistic Pleiotropic Gene Action for Mortality and Advanced Age

Association or linkage studies involving control and long-lived populations provide information on genes that influence longevity. However, the relationship between allele-specific differences in survival and the genetic structure of aging cohorts remains unclear. We model a heterogeneous cohort comprising several genotypes differing in age-specific mortality. In its most general form, without any specific assumption regarding the shape of mortality curves, the model permits derivation of a fundamental property underlying abrupt age-related changes in the composition of a cohort. The model is applied to sex-specific survival curves taken from period life tables, and Gompertz-Makeham mortality coefficients are calculated for the French population. Then, adjustments are performed under Gompertz-Makeham mortality functions for three genotypes composing a heterogeneous cohort, under the constraint of fitting the resultant mortality to the real French population mortality obtained from life tables. Multimodal curves and divergence after the 8th decade appear as recurrent features of the frequency trajectories. Finally, a fit to data previously obtained at the angiotensin-converting-enzyme locus is realized, explaining what had seemed to be paradoxical results-namely, that the frequency of a genotype known as a cardiovascular risk factor was increased in centenarians. Our results help explain the well-documented departure from Gompertz-Makeham mortality kinetics at older ages. The implications of our model are discussed in the context of known genetic effects on human longevity and age-related pathologies. Since antagonistic pleiotropy between early and late survival emerges as a general rule, extrapolating the effects measured for a gene in a particular age class to other ages could be misleading.     

POLLEN APERTURE POLYMORPHISM AND GAMETOPHYTE PERFORMANCE IN VIOLA DIVERSIFOLIA

Pollen aperture polymorphism is studied in Viola diversifolia, where all plants produce three- and four-apertured pollen grains. We tested whether there are genetic differences among plants for the proportions of the different pollen morphs, and whether the morphs differ in gametophytic performance. Results show that the more apertures a pollen grain has, the more quickly it germinates but that few-apertured pollen grains have faster growing pollen tubes and longer life expectancies. The proportions of the different pollen morphs, together with pollen tube growth rates, may be inherited traits based on differences among maternal families. These results suggest that the different pollen morphs are favored in different pollination ecology situations. The production of several pollen morphs by the same individual could therefore be evolutionarily advantageous.     

Function of Lysosomes and Lysosomal Enzymes in the Senescing Corolla of the Morning Glory (Ipomoea purpurea)

The rapid senescence of the Ipomoea corolla is characterizedby the breakdown of protein and nucleic acids. At the onsetof wilting the activities of deoxyribonuclease (DNase), ribonuclease(RNase), and ß-glucosidase are increased dramatically,while other hydrolytic activities such as the actions of protease,aminopeptidase, -glucosidase, phosphatase, esterase, and -amylaseare only slightly changed. Isolated corolla discs show a course of senescence similar tothat of the intact organ. When floating on solutions of cycloheximidethe activities of DNase, RNase, and ß-glucosidasedo not increase. Actinomycin D inhibits the increase in RNaseactivity. It is concluded that protein synthesis is a prerequisitefor the changes in these enzyme activities in the senescingcorolla. The function of the lysosomal compartment in the process ofsenescence is illustrated by electron micrographs showing theautophagic activity of vacuoles. The last phase of senescenceis characterized by the breakdown of the tonoplast and completedigestion of the cytoplasmic constituents in the autolysingcells.