Paternal-specific S-allele transmission in sweet cherry (Prunus avium L.): the potential for sexual selection.

Homomorphic self-incompatibility is a well-studied example of a physiological process that is thought to increase population diversity and reduce the expression of inbreeding depression. Whereas theoretical models predict the presence of a large number of S-haplotypes with equal frequencies at equilibrium, unequal allele frequencies have been repeatedly reported and attributed to sampling effects, population structure, demographic perturbation, sheltered deleterious mutations or selection pressure on linked genes. However, it is unclear to what extent unequal segregations are the results of gametophytic or sexual selection. Although these two forces are difficult to disentangle, testing S-alleles in the offspring of controlled crosses provides an opportunity to separate these two phenomena. In this work, segregation and transmission of S-alleles have been characterized in progenies of mixed donors and fully compatible pollinations under field conditions in Prunus avium. Seed set patterns and pollen performance have also been characterized. The results reveal paternal-specific distorted transmission of S-alleles in most of the crosses. Interestingly, S-allele segregation within any given paternal or maternal S-locus was random. Observations on pollen germination, pollen tube growth rate, pollen tube cohort size, seed set dynamics and transmission patterns strongly suggest post-pollination, prezygotic sexual selection, with male-male competition as the most likely mechanism. According to these results, post-pollination sexual selection takes precedence over frequency-dependent selection in explaining unequal S-haplotype frequencies.

Pollen performance, cell number, and physiological state in the early-divergent angiosperm Annona cherimola Mill. (Annonaceae) are related to environmental conditions during the final stages of pollen development.

Pollen performance is an important determinant for fertilization success, but high variability in pollen behavior both between and within species occurs in different years and under varying environmental conditions. Annona cherimola, an early-divergent angiosperm, is a species that releases a variable ratio of bicellular and tricellular hydrated pollen at anther dehiscence depending on temperature. The presence of both bi- and tricellular types of pollen is an uncommon characteristic in angiosperms and makes Annona cherimola an interesting model to study the effect of varying environmental conditions on subsequent pollen performance during the final stages of pollen development. In this work, we study the influence of changes in temperature and humidity during the final stages of pollen development on subsequent pollen performance, evaluating pollen germination, presence of carbohydrates, number of nuclei, and water content. At 25 °C, which is the average field temperature during the flowering period of this species, pollen had a viability of 60-70 %, starch hydrolyzed just prior to shedding, and pollen mitosis II was taking place, resulting in a mixture of bi- and tricellular pollen. This activity may be related to the pollen retaining 70 % water content at shedding. Temperatures above 30 °C resulted in a decrease in pollen germination, whereas lower temperatures did not have a clear influence on pollen germination, although they did have a clear effect on starch hydrolysis. On the other hand, slightly higher dehydration accelerated mitosis II, whereas strong dehydration arrested starch hydrolysis and reduced pollen germination. These results show a significant influence of environmental conditions on myriad pollen characteristics during the final stages of pollen development modifying subsequent pollen behavior and contributing to our understanding of the variability observed in pollen tube performance.

Self-compatibility in 'Cristobalina' sweet cherry is not associated with duplications or modified transcription levels of S-locus genes.

Sweet cherry shows S-RNase-based gametophytic self-incompatibility, which prevents self- and cross-fertilization between genetically related individuals. The specificity of the self-incompatible reaction is determined by two genes located in the S-locus. These encode a pistil-expressed ribonuclease (S-RNase) that inhibits self pollen tube growth, and a pollen-expressed F-box protein (SFB) that may be involved in the cytotoxicity of self-S-RNases. Initial genetic and pollination studies in a self-compatible sweet cherry cultivar, 'Cristobalina' (S (3) S (6)), showed that self-compatibility was caused by the loss of pollen function of both haplotypes (S (3) and S (6)). In this study, we further characterize self-compatibility in this genotype by molecular analysis of the S-locus. DNA blot analyses using S-RNase and SFB probes show no duplications of 'Cristobalina' S-locus genes or differences in the restriction patterns when compared with self-incompatible cultivars with the same S-genotype. Furthermore, reverse transcriptase-PCR of S-locus genes and quantitative reverse transcription-PCR of SFBs revealed no differences at the transcription level when compared with a self-incompatible genotype. The results of this study show that no differences at the S-locus can be correlated with self-compatibility, indicating the possible involvement of non-S-locus modifiers in self-incompatibility breakdown in this cultivar.

Alternative food improves the combined effect of an omnivore and a predator on biological pest control. A case study in avocado orchards.

Ecological communities used in biological pest control are usually represented as three-trophic level food chains with top-down control. However, at least two factors complicate this simple way of characterizing agricultural communities. First, agro-ecosystems are composed of several interacting species forming complicated food webs. Second, the structure of agricultural communities may vary in time. Efficient pest management approaches need to integrate these two factors to generate better predictions for pest control. In this work, we identified the food web components of an avocado agro-ecosystem, and unravelled patterns of co-occurrence and interactions between these components through field and laboratory experiments. This allowed us to predict community changes that would improve the performance of the naturally occurring predators and to test these predictions in field population experiments. Field surveys revealed that the food-web structure and species composition of the avocado community changed in time. In spring, the community was characterized by a linear food chain of Euseius stipulatus, an omnivorous mite, feeding on pollen. In the summer, E. stipulatus and a predatory mite, Neoseiulus californicus, shared a herbivorous mite prey. Laboratory experiments confirmed these trophic interactions and revealed that N. californicus can feed inside the prey nests, whereas E. stipulatus cannot, which may further reduce competition among predators. Finally, we artificially increased the coexistence of the two communities via addition of the non-herbivore food source (pollen) for the omnivore. This led to an increase in predator numbers and reduced populations of the herbivore. Therefore, the presence of pollen is expected to improve pest control in this system.

Influence of genotype-temperature interaction on pollen performance.

Pollen competition and selection have significant evolutionary consequences, but very little is known about how they can be modulated. We have examined in cherry (Prunus avium L.) how pollen performance is affected by the genotype of the pollen and by the environmental conditions under which it grows, namely the pistilar tissue and temperature. The different pollen donor genotypes tested in this work differed in their behaviour both in vitro and in vivo and this behaviour was modulated depending on the female recipient they grew on. Furthermore, there was a significant temperature-genotype interaction that affected the pollen tube population census that succeeded in reaching the base of the style. The combination of these three factors, while enabling a capacity of response to variations in environmental pressures, could maintain variability in pollen performance avoiding the fixation of the genes that control pollen tube growth rate.