Contrasting reproductive strategies of triploid hybrid males in vertebrate mating systems.

The scarcity of parthenogenetic vertebrates is often attributed to their 'inferior' mode of clonal reproduction, which restricts them to self-reproduce their own genotype lineage and leaves little evolutionary potential with regard to speciation and evolution of sexual reproduction. Here, we show that for some taxa, such uniformity does not hold. Using hybridogenetic water frogs (Pelophylax esculentus) as a model system, we demonstrate that triploid hybrid males from two geographic regions exhibit very different reproductive modes. With an integrative data set combining field studies, crossing experiments, flow cytometry and microsatellite analyses, we found that triploid hybrids from Central Europe are rare, occur in male sex only and form diploid gametes of a single clonal lineage. In contrast, triploid hybrids from north-western Europe are widespread, occur in both sexes and produce recombined haploid gametes. These differences translate into contrasting reproductive roles between regions. In Central Europe, triploid hybrid males sexually parasitize diploid hybrids and just perpetuate their own genotype--which is the usual pattern in parthenogens. In north-western Europe, on the other hand, the triploid males are gamete donors for diploid hybrids, thereby stabilizing the mixed 2n-3n hybrid populations. By demonstrating these contrasting roles in male reproduction, we draw attention to a new significant evolutionary potential for animals with nonsexual reproduction, namely reproductive plasticity.

Effects of geographic distance, sea barriers and habitat on the genetic structure and diversity of all-hybrid water frog populations.

The history of population size and migration patterns leaves its mark in the genetics of populations. We investigate the genetic structure of the edible frog, Pelophylax esculentus in the Danish archipelago and adjacent countries. This frog is of particular interest because it is a hybrid that, in this area, forms all-hybrid populations of diploid (LR) and triploid (LLR and LRR) genomotypes with no (or very few) adults of the parental species (LL and RR). This study is the first to cover the entire geographic range of Danish, Swedish and German all-hybrid populations, documenting their extent and providing a broad picture of their diversity of neutral genetic markers and genomotype proportions. With 18 microsatellite markers, we found that genetic diversity declines northwards in agreement with the glacial refuge and central-marginal hypotheses; however, populations on small and medium-sized islands are no less diverse than those on large islands and continental peninsulas. Isolation by distance exists across the archipelago with limited influence of fragmentation by brackish seawater. The extremely low genetic diversity in all-hybrid populations, compared with adjacent populations, may be responsible for the maintenance of their special breeding system. We also show large variation among ponds in proportions of LLR, LR and LRR genomotypes, but little geographic pattern in their distribution. Instead, we found relationships between the genomotype proportions and some of 15 habitat parameters monitored. Body size differences among LLR, LR and LRR further suggest ecological differences.

Widespread unidirectional transfer of mitochondrial DNA: a case in western Palaearctic water frogs.

Interspecies transfer of mitochondrial (mt) DNA is a common phenomenon in plants, invertebrates and vertebrates, normally linked with hybridization of closely related species in zones of sympatry or parapatry. In central Europe, in an area north of 48 degrees N latitude and between 8 degrees and 22 degrees E longitude, western Palaearctic water frogs show massive unidirectional introgression of mtDNA: 33.7% of 407 Rana ridibunda possessed mtDNA specific for Rana lessonae. By contrast, no R. lessonae with R. ridibunda mtDNA was observed. That R. ridibunda with introgressed mitochondrial genomes were found exclusively within the range of the hybrid Rana esculenta and that most hybrids had lessonae mtDNA (90.4% of 335 individuals investigated) is evidence that R. esculenta serves as a vehicle for transfer of lessonae mtDNA into R. ridibunda. Such introgression has occurred several times independently. The abundance and wide distribution of individuals with introgressed mitochondrial genomes show that R. lessonae mt genomes work successfully in a R. ridibunda chromosomal background despite their high sequence divergence from R. ridibunda mtDNAs (14.2-15.2% in the ND2/ND3 genes). Greater effectiveness of enzymes encoded by R. lessonae mtDNA may be advantageous to individuals of R. ridibunda and probably R. esculenta in the northern parts of their ranges.

Hemiclonal reproduction slows down the speed of Muller's ratchet in the hybridogenetic frog Rana esculenta.

Rare recombination in otherwise asexually reproducing organisms is known to beneficially influence the fitness in small populations. In most of the investigated organisms, asexual and rare sexual generations with recombination follow each other sequentially. Here we present a case where clonal reproduction and rare recombination occur simultaneously in the same population. The hybridogenetic water frog Rana esculenta (E), a hybrid between R. lessonae (L) and R. ridibunda (R) produces gametes that only contain the unaltered maternal R part of their genome. New generations of R. esculenta usually arise from E x L matings. Intraspecific E x E matings produce mostly inviable offspring, but in rare cases, female R. ridibunda arise from such matings which are capable of recombination. In the absence of conspecific males, these R females have to mate with E males, which results in further R females, or with L males, which produces new E lineages. This indirect mechanism reintroduces recombination into the otherwise clonally transmitted R genomes in R. esculenta populations. In this study, we show through Monte Carlo simulations that, in most cases, it is sufficient that only between 1 % and 10 % of mixed water frog populations consist of R females to prevent or significantly reduce the fixation and accumulation of deleterious mutations.

Hibernation behavior of Rana lessonae and R. esculenta in their natural habitat.

We studied the hibernation behavior of the water frog Rana lessonae and its hybridogenetic associate R. esculenta in their natural habitat during three successive winters. Animals caught in pitfall traps at a fenced pond were individually marked with PIT tags and some (n=36) were additionally equipped with radio transmitters. Of the animals caught, 85% left the fenced pond for hibernation. More R. esculenta remained inside the fenced area compared to R. lessonae. R. lessonae emigrated earlier in autumn and came back later in spring than R. esculenta, but the distance to their hibernation sites did not differ. Both species left the fenced pond earlier in the year when ambient temperatures were lower. All radio-tracked animals hibernated in woodland, 3-7 cm below the surface in soil, under moss, fallen leaves or small branches. Soil temperatures at the actual hibernation sites were significantly higher than at randomly chosen control sites. A surprising finding was that most frogs changed their hibernation sites during winter, and often more than once. Movements were more frequent in the warmer first half of the winter than in the cooler second half, but some animals were active even on days with mean temperatures below 1°C. These results show that both species do not spend the whole winter torpid in one particular hibernation site but move around, especially at higher temperatures. Most of the animals lost weight during the winter, and the weight loss was greater in females than in males and higher in warm than in cold winters. To what extent weight loss and survival is influenced by the chosen hibernation sites and the amount of movement during winter, and whether this contributes to the differences in species and sex ratios found in mixed populations, needs more investigation.

Cryptic female choice: frogs reduce clutch size when amplexed by undesired males.

In species with internal fertilization, females can 'cryptically' choose (e.g. through sperm selection) which individuals sire their offspring, even when their overt preferences for copulatory partners are overrun by male-male competition and sexual coercion. The experiment presented here reveals that control of paternity after copulation has begun is also possible in species with external fertilization. Females of the hybridogenetic Rana essonae-Rana esculenta (LL-LR) waterfrog complex adjust their clutch size in response to mate type: they release fewer eggs when amplexed by hybrid LR males who--jeopardize successful reproduction--than when amplexed by parental LL males. This reduction in the number of eggs laid can increase a female's residual reproductive value through a second mating in the same breeding season or a larger clutch size in the next year. We argue that cryptic female choice through clutch size adjustment (i) may have evolved more often than previously assumed, and (ii) can arise even where females mate only once during a reproductive period.

The importance of food quantity and quality for reproductive performance in alpine water pipits (Anthus spinoletta).

Studies relating reproduction to food availability are usually restricted to food quantity, but ignore food quality and the effects of habitat structure on obtaining the food. This is particularly true for insectivorous birds. In this study we relate measures of reproductive success, time of reproduction and nestling size of water pipits (Anthus spinoletta) to biomass, taxonomic composition and nutritional content of available food, and to vegetation structure and distance to feeding sites. Clutch size was positively correlated with the proportion of grass at the feeding sites, which facilitates foraging. This suggests that water pipits adapt their clutch size to environmental conditions. Also, pipits started breeding earlier and produced more fledglings when abundant food and a large proportion of grass were available, probably because these conditions allow the birds to gain more energy in less time. The number of fledglings was positively correlated with the energy content of available food. No significant relationships were found between feeding conditions and nestling size or the time that nestlings took to fledge. This suggests that water pipits do not invest more in individual nestlings when food conditions are favourable but rather start breeding earlier and produce more young. Taxonomic composition and nutritional content of prey were not correlated with any of the reproductive parameters, indicating that profitability rather than quality of food affects reproductive success.

Honeyguides and honey gatherers: interspecific communication in a symbiotic relationship.

In many parts of Africa, people searching for honey are led to bees' nests by the greater honeyguide (Indicator indicator Sparrman). The Boran people of Kenya claim that they can deduce the direction and the distance to the nest as well as their own arrival at the nest from the bird's flight pattern, perching height, and calls. Analyses of the behavior of guiding birds confirmed these claims.

Nectar intake and energy expenditure in a flower visiting bat.

In a coastal region of Venezuela the daily energy expenditure (DEE) and water turnover of the flower visiting bat Anoura caudifer was measured by using the doubly labeled water method. In flower visitors, this method allows independent measurement of energy intake and expenditure if the animals drink no additional water and if the nectar's energy content is known. An average DEE of 12.4 kcal/d and water exchange of 13.4 ml/d were found. Our data show a balanced energy budget when animals in the field imbibe nectar with a sugar concentration of 18-21%, which is roughly medial in the range of nectar concentrations of various bat flowers. The energy turnover of flower visiting bats is high compared with DEEs of other bat species, small mammals and birds; flower visiting bats seem to belong to those species having 'a fast spin of the life motor'.

[Origins and consequences of aggression in Etroplus maculatus (Cichlidae, Pisces). The problem of drive]. / Ursachen und Konsequenzen von Aggressivität bei Etroplus maculatus (Cichlidae, Pisces)

Increased aggressive activity between male and female in isolated pairs of Etroplus maculatus has hitherto been explained either by the drive hypothesis or by the external stimulus hypothesis. In an attempt to investigate these two hypotheses the partners of 48 pairs were separated and their aggressiveness towards their mirrored images and their non-aggressive activities were recorded the following day and again 4 days later after 180 brief presentations of an adjacent compartment. For both sexes aggressiveness was lowered if this compartement was empty (isolation) and heightened if it held the original mate. Females also reacted more aggressively on seeing conspecific strangers of both sexes. There was no significant alteration of non-aggressive activities in any of the series. The increase of aggressiveness caused by the presence of the mate was normally offset by various investigated mechanisms preventing its eruption against the partner. Attacks only occurred if the equilibrium between aggressiveness and inhibition was disturbed. All results and those of other authors are discussed; it is concluded that the behaviour of E. maculatus can be better explained in terms of external stimulation than in terms of an endogenous production of aggressive drive energy.