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. Received: 4 August 1999 / Accepted: 15 November 1999     

Relationships between food resources, foraging patterns, and reproductive success in the water pipit, Anthus sp. spinoletta

A basic but rarely tested assumption in optimal foraging theoryis that positive relationships exist between the foraging patternof an animal, its short-term benefits in feeding, and its long-termfitness. We present evidence for these relationships for a centralplace foraging situation. We studied the foraging behavior ofadult water pipits (Anthus sp. spinoletta) feeding nestlingsin an Alpine habitat near Davos, Switzerland, with the followingresults: (1) searching effort decreases with increasing distancefrom the nest, (2) the amount of prey and the proportion oflarge items brought to the nest increases with increasing foragingdistance, (3) water pipits do not forage according to habitatavailability, but prefer vegetation types with the highest fooddensity (mainly grass and herbs) and avoid those with the lowest,and (4) this selectivity is only expressed when the birds foragemore than 50 m from the nest, i.e., usually outside the territory.Among the several potential interpretations of these results,the most parsimonious is that foraging decisions are based onprofitability, i.e., on the net energy gain per time unit. Additionally,we found that food conditions translate into fitness: the numberof fledglings per nest is related positively to the averageprey biomass at the foraging place and negatively to the averagedistance between the foraging place and the nest. Maximum economicdistances, which were predicted from this food-fitness relationship,agreed well with the actual foraging distances observed. Thissuggests a dose connection between foraging decisions and fitness.In addition to the theoretical issues, some conservation issuesare also briefly discussed.     

Expression variation of the porcine ADRB2 has a complex genetic background

Porcine adrenergic receptor beta 2 (ADRB2) gene exhibits differential allelic expression in skeletal muscle, and its genetic variation has been associated with muscle pH. Exploring the molecular–genetic background of expression variation for porcine ADRB2 will provide insight into the mechanisms driving its regulatory divergence and may also contribute to unraveling the genetic basis of muscle-related traits in pigs. In the present study, we therefore examined haplotype effects on the expression of porcine ADRB2 in four tissues: longissimus dorsi muscle, liver, subcutaneous fat, and spleen. The diversity and structure of haplotypes of the proximal gene region segregating in German commercial breeds were characterized. Seven haplotypes falling into three clades were identified. Two clades including five haplotypes most likely originated from introgression of Asian genetics during formation of modern breeds. Expression analyses revealed that the Asian-derived haplotypes increase expression of the porcine ADRB2 compared to the major, wild-type haplotype independently of tissue type. In addition, several tissue-specific differences in the expression of the Asian-derived haplotypes were found. Inspection of haplotype sequences showed that differentially expressed haplotypes exhibit polymorphisms in a polyguanine tract located in the core promoter region. These findings demonstrate that expression variation of the porcine ADRB2 has a complex genetic basis and suggest that the promoter polyguanine tract is causally involved. This study highlights the challenges of finding causal genetic variants underlying complex traits.     

Gamete production patterns,ploidy, and population genetics reveal evolutionary significant units in hybrid water frogs (Pelophylax esculentus)

The European water frog Pelophylax esculentus is a natural hybrid between P. lessonae (genotype LL) and P. ridibundus (RR). It reproduces through hybridogenesis, eliminating one parental genome from its germline and producing gametes containing the genome of the other parental species. According to previous studies, this elimination and transmission pattern is very diverse. In mixed populations, where only diploid hybrids (LR) live in sympatry and mate with one or both parental species, the excluded genome varies among regions, and the remaining genome is transmitted clonally to haploid gametes. In all‐hybrid populations consisting of diploid (LR) and triploid (LLR and/or LRR) frogs, diploid individuals also produce gametes clonally (1n in males, 2n in females), whereas triploids eliminate the genome they have in single copy and produce haploid gametes containing the recombined other genome. However, here, too, regional differences seem to exist, and some triploids have been reported to produce diploid gametes. In order to systematically study such regional and genotype differences in gamete production, their potential origin, and their consequences for the breeding system, we sampled frogs from five populations in three European countries, performed crossing experiments, and investigated the genetic variation through microsatellite analysis. For four populations, one in Poland, two in Germany, and one in Slovakia, our results confirmed the elimination and transmission pattern described above. In one Slovakian population, however, we found a totally different pattern. Here, triploid males (LLR) produce sperm with a clonally transmitted diploid LL genome, rather than a haploid recombined L genome, and LR females clonally produce haploid R eggs, rather than diploid LR eggs. These differences among the populations in gamete production go along with differences in genomotype composition, breeding system (i.e., the way triploids are produced), and genetic variation. These differences are strong evidence for a polyphyletic origin of triploids. Moreover, our findings shed light on the evolutionary potential inherent to the P. esculentus complex, where rare events due to untypical gametogenetic processes can lead to the raise, the perpetuation, and the dispersion of new evolutionary significant lineages which may also deserve special conservation measures.     

Choosy females and indiscriminate males: mate choice in mixed populations of sexual and hybridogenetic water frogs (Rana lessonae, Rana esculenta)

For several decades, behavioral ecologists have studied theeffects of the environment on the behavior of individuals;but only fairly recently they have started to ask the reversequestion: how do the behavioral strategies of individuals affectthe composition and dynamics of populations and communities?Although intuitively obvious, this feedback from individualto higher levels is difficult to demonstrate, except in systemswith exceptionally fast and marked responses of the populationsto the behavior of its members. Such a system exists in sperm-dependentspecies. In European water frogs, for instance, successfulreproduction of a hybrid species (R. esculenta, genotype LR)requires mating with one of its parental species (R. lessonae,genotype LL), except in the rare cases where hybrids are triploid.The sexual host LL, however, should avoid matings with the sexual parasite LR, because the resulting LR offspring willeliminate the L genome from their germ line. In this studywe investigate how this conflict is solved. Since water froghybrids come in both sexes, rather than as females only likein other sperm-dependent systems, we performed the tests withboth females and males. One individual was given a choice betweentwo individuals of the opposite sex, one an LL and the otheran LR. In both species, females showed the predicted preferencefor LL males, whereas males did not discriminate between LLand LR females. On the individual level, we interpret the sexdifference in choosiness by the lower costs from mating withthe wrong species (LR) and the higher benefits from matingwith large individuals in males than in females. In "normal"species, male preference for large (i.e. more fecund) femalesis advantageous, but in this system such a choice can resultin mating with the larger LR females. With respect to the structureand dynamics of mixed populations, we discuss that the observed female preference is consistent with the higher mating successof LL males found in nature. Hence, mate female choice is astrong candidate for a mechanism promoting coexistence of thesperm-dependent hybrid and its sexual host. This confirms predictionsfrom previous theoretical models.     

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.     

Mutation accumulation and fitness effects in hybridogenetic populations: a comparison to sexual and asexual systems

Background  

Female only unisexual vertebrates that reproduce by hybridogenesis show an unusual genetic composition. They are of hybrid origin but show no recombination between the genomes of their parental species. Instead, the paternal genome is discarded from the germline prior to meiosis, and gametes (eggs only) contain solely unrecombined maternal genomes. Hence hybridogens only transmit maternally inherited mutations. Hybridity is restored each generation by backcrossing with males of the sexual parental species whose genome was eliminated. In contrast, recombining sexual species propagate an intermixed pool of mutations derived from the maternal and paternal parts of the genome. If mutation rates are lower in female gametes than males, it raises the possibility for lower mutation accumulation in a hybridogenetic population, and consequently, higher population fitness than its sexual counterpart.     

Genome‐wide SNP analysis unveils genetic structure and phylogeographic history of snow sheep (Ovis nivicola) populations inhabiting the Verkhoyansk Mountains and Momsky Ridge (northeastern Siberia)

Insights into the genetic characteristics of a species provide important information for wildlife conservation programs. Here, we used the OvineSNP50 BeadChip developed for domestic sheep to examine population structure and evaluate genetic diversity of snow sheep (Ovis nivicola) inhabiting Verkhoyansk Range and Momsky Ridge. A total of 1,121 polymorphic SNPs were used to test 80 specimens representing five populations, including four populations of the Verkhoyansk Mountain chain: Kharaulakh Ridge–Tiksi Bay (TIK, n = 22), Orulgan Ridge (ORU, n = 22), the central part of Verkhoyansk Range (VER, n = 15), Suntar‐Khayata Ridge (SKH, n = 13), and Momsky Ridge (MOM, n = 8). We showed that the studied populations were genetically structured according to a geographic pattern. Pairwise F_ST values ranged from 0.044 to 0.205. Admixture analysis identified K = 2 as the most likely number of ancestral populations. A Neighbor‐Net tree showed that TIK was an isolated group related to the main network through ORU. TreeMix analysis revealed that TIK and MOM originated from two different ancestral populations and detected gene flow from MOM to ORU. This was supported by the f3 statistic, which showed that ORU is an admixed population with TIK and MOM/SKH heritage. Genetic diversity in the studied groups was increasing southward. Minimum values of observed (Ho) and expected (He) heterozygosity and allelic richness (Ar) were observed in the most northern population—TIK, and maximum values were observed in the most southern population—SKH. Thus, our results revealed clear genetic structure in the studied populations of snow sheep and showed that TIK has a different origin from MOM, SKH, and VER even though they are conventionally considered a single subspecies known as Yakut snow sheep (Ovis nivicola lydekkeri). Most likely, TIK was an isolated group during the Late Pleistocene glaciations of Verkhoyansk Range.     

The Relative Importance of Habitat Structure and of Prey Characteristics for the Foraging Success of Water Pipits (Anthus spinoletta)*

While many studies on foraging have related energy gain to the density and the size of prey, only few have investigated whether and how habitat structure modifies the gain through affecting foraging success. In this study, the influences of habitat structure and prey characteristics on the foraging success of water pipits, Anthus spinoletta, were investigated experimentally. The birds take longer to find prey in tall than in short vegetation. The effects of vegetation on searching times differ between prey types. These differences are probably caused by variation in prey behaviour and in cryptic colouration, but not by prey size. Searching times increase with decreasing density for mealworms and tipulids, but not for caterpillars. Handling large prey items requires more time than handling smaller prey. Tipulids and caterpillars, which were offered alive, are handled for a longer time than dead mealworms of corresponding size. The success of attacks on flying insects is probably influenced by the prey's flight speed: fast houseflies are missed more often than slow tipulids. Overall, the results show that the time costs of foraging water pipits are influenced to a comparable degree by vegetation structure, by prey density and by other specific prey characteristics such as camouflage, hiding behaviour or agility. The amount of food gathered per unit time is determined primarily by factors that affect searching times, and less by handling and travelling times. Insertion of our data into an optimal diet model leads to the prediction that water pipits should be generalist foragers, which agrees with the observed behaviour.