The effect of antioxidants on sperm motility activation in the Booroolong frog.

Motile sperm can generate high levels of reactive oxygen species (ROS) post activation, and ROS can quickly accumulate to levels that impair motility and fertilising ability. The addition of antioxidants to sperm suspensions has been suggested as a means of reducing oxidative stress and enhancing sperm motility during and after sperm storage. Despite this, very few studies have attempted to experimentally test the effects of antioxidants on sperm motility activation in animals that use an external mode of fertilisation, espcially in amphibians. The present study aimed to investigate the effect of vitamin C and vitamin E on sperm motility activation in the Booroolong frog. Spermatozoa were activated in media containing either vitamin C (0, 0.05, 0.10, 0.15, 0.20, 0.25µgµL-1) or vitamin E (0, 0.25, 0.50, 0.75, 1.0, 1.25 1.50, 1.75µgµL-1). Sperm performance parameters (percent motility and velocity) were assessed using CASA at 0, 1, 2, 3, 4, 5 and 6h post-activation. Contrary to expectations, vitamin C supplementation was detrimental to sperm motility across all tested concentrations, while vitamin E had no effect. Further investigation on the endogenous antioxidant system of anuran sperm is required to ascertain whether alternative antioxidants may be more suitable at reducing ROS produced during sperm activation and improving sperm motility activation in vitro.

Environmental osmolality influences sperm motility activation in an anuran amphibian.

Evolutionary theory predicts that selection will favour sperm traits that maximize fertilization success in local fertilization environments. In externally fertilizing species, osmolality of the fertilization medium is known to play a critical role in activating sperm motility, but there remains limited evidence for adaptive responses to local osmotic environments. In this study, we used a split-sample experimental design and computer-assisted sperm analysis to (i) determine the optimal medium osmolality for sperm activation (% sperm motility and sperm velocity) in male common eastern froglets (Crinia signifera), (ii) test for among-population variation in percentage sperm motility and sperm velocity at various activation-medium osmolalities and (iii) test for among-population covariation between sperm performance and environmental osmolality. Frogs were obtained from nine populations that differed in environmental osmolality, and sperm samples of males from different populations were subjected to a range of activation-medium osmolalities. Percentage sperm motility was optimal between 10 and 50 mOsm kg(-1) , and sperm velocity was optimal between 10 and 100 mOsm kg(-1) , indicating that C. signifera has evolved sperm that can function across a broad range of osmolalities. As predicted, there was significant among-population variation in sperm performance. Furthermore, there was a significant interaction between activation-medium osmolality and environmental osmolality, indicating that frogs from populations with higher environmental osmolality produced sperm that performed better at higher osmolalities in vitro. This finding may reflect phenotypic plasticity in sperm functioning, or genetic divergence resulting from spatial variation in the strength of directional selection. Both of these explanations are consistent with evolutionary theory, providing some of the first empirical evidence that local osmotic environments can favour adaptive sperm motility responses in species that use an external mode of fertilization.

The role of phylogeny and ecology in shaping morphology in 21 genera and 127 species of Australo-Papuan myobatrachid frogs.

Body shape is predicted to differ among species for functional reasons and in relation to environmental niche and phylogenetic history. We quantified morphological differences in shape and size among 98.5% of the 129 species and all 21 genera of the Australo-Papuan endemic myobatrachid frogs to test the hypothesis that habitat type predicts body shape in this radiation. We tested this hypothesis in a phylogenetic context at two taxonomic levels: across the entire radiation and within the four largest genera. Thirty-four external measurements were taken on 623 museum specimens representing 127 species. Data for seven key environmental variables relevant to anurans were assembled for all Australian-distributed species based on species' distributions and 131,306 locality records. The Australo-Papuan myobatrachid radiation showed high diversity in adult body size, ranging from minute (15 mm snout-vent length) to very large species (92 mm), and shape, particularly sin relative limb length. Five main morphological and environmental summary variables displayed strong phylogenetic signal. There was no clear relationship between body size and environmental niche, and this result persisted following phylogenetic correction. For most species, there was a better match between environment/habitat and body shape, but this relationship did not persist following phylogenetic correction. At a broad level, species fell into three broad groups based on environmental niche and body shape: 1) species in wet habitats with relatively long limbs, 2) species in arid environments with relatively short limbs (many of which are forward or backward burrowers) and 3) habitat generalist species with a conservative body shape. However, these patterns were not repeated within the four largest genera - Crinia, Limnodynastes, Pseudophryne and Uperoleia. Each of these genera displayed a highly conservative anuran body shape, yet individual species were distributed across the full spectrum of Australian environments. Our results suggest that phylogenetic legacy is important in the evolution of body size and shape in Australian anurans, but also that the conservative body plan of many frogs works well in a wide variety of habitats.

Remating in Drosophila melanogaster: an examination of the trading-up and intrinsic male-quality hypotheses.

Female Drosophila melanogaster remate more frequently than necessary to ensure fertilization. We tested whether polyandrous females gain genetic benefits for their offspring by (1) selecting secondary sires of higher genetic-quality than original partners or (2) because post-copulatory mechanisms bias fertilizations towards genetically superior males. We screened 119 hemiclones of males for lifetime fitness then selected eight hemiclones (four of extreme high fitness and four of extreme low fitness) and mated them to virgin females. Females were then given the opportunity to remate with males of benchmark-genetic quality and their propensity to remate (fidelity) and sperm displacement scored. A female's fidelity and her level of sperm displacement varied depending on which hemiclone she mated first, but not on male-genetic quality. These findings indicate that female remating and sperm displacement are strongly influenced by male genotype, but provide no evidence that these traits contribute to adaptive female choice to obtain superior genes for offspring.

Does multiple paternity improve fitness of the frog Crinia georgiana?

In the Australian myobatrachid frog Crinia georgiana simultaneous polyandry occurs in about half of all matings, which leads to multiple paternity, but reduced fertilization success and occasional female mortality. Multiple paternity may provide benefits to females that compensate for these costs, for example, through enhanced genetic diversity of a clutch. In nature, embryos and tadpoles of C. georgiana develop in shallow, temporary pools and may be exposed to fluctuating water levels and the risk of desiccation between rain events. Fertilization by genetically diverse sires may act as a bet hedge against these conditions. To evaluate this hypothesis, females were artificially mated with one or two males in the field and eggs and larvae reared in the laboratory under constant or fluctuating developmental conditions. Experiment 1 exposed embryos from single- and multiple-paternity clutches to conditions where eggs were completely covered during development or eggs sat in air on a moist substrate. Experiment 2 exposed freshly hatched larvae from single- and multiple-paternity clutches to constant wet conditions, where larvae were completely covered, or fluctuating wet conditions, where larvae ranged from being completely submersed to partially exposed over a 13-day cycle. We measured mean performance and best performance as alternate measures of genetic benefits. There were no effects of paternity on percent survival to hatching, time to hatching, body size at hatching, percent survival to metamorphosis, time to metamorphosis, or body size at metamorphosis. We also analyzed variance within clutches as a measure of genetic diversity. Again there were no predictable effects of multiple paternity. Polyandry does not appear to provide any genetic benefits that compensate for the high costs of polyandry in this species.