Circular DNA Intermediate in the Duplication of Nile Tilapia vasa Genes

vasa is a highly conserved RNA helicase involved in animal germ cell development. Among vertebrate species, it is typically present as a single copy per genome. Here we report the isolation and sequencing of BAC clones for Nile tilapia vasa genes. Contrary to a previous report that Nile tilapia have a single copy of the vasa gene, we find evidence for at least three vasa gene loci. The vasa gene locus was duplicated from the original site and integrated into two distant novel sites. For one of these insertions we find evidence that the duplication was mediated by a circular DNA intermediate. This mechanism of gene duplication may explain the origin of isolated gene duplicates during the evolution of fish genomes. These data provide a foundation for studying the role of multiple vasa genes in the development of tilapia gonads, and will contribute to investigations of the molecular mechanisms of sex determination and evolution in cichlid fishes

Sex differentiation of channel catfish gonads: Normal development and effects of temperature

Channel catfish have an XX female-XY male sex determination system. Although genetic males of this species are readily feminized with exogenous steroid, the pattern of gonadal sex differentiation and the effect of temperature on genetic sex determination are unknown. To document the pattern of gonadal sex differentiation, catfish from a mixed-sex progeny (XX female-XY male cross), known XY males (XX female-YY male cross), and steroid sex-reversed (XY) females were reared at 28°C and collected for histological analysis from Day 7 to Day 90 postfertilization. No signs of gonadal sex differentiation were detected from Day 7 to Day 16. On Day 19, about half the fish from the mixed-sex population as well as the sex- reversed females had gonads with proximal and distal tissue outgrowths. These outgrowths eventually fused to form an ovarian cavity. Germ cell meiosis in these gonads was first detected on Day 22 and growing ovarian follicles were seen at later stages. Thus, ovarian differentiation began about Day 19. Although presumptive testes lose their sensitivity to feminizing steroids about Day 19, the gonads of known males and of putative males from the mixed-sex population showed no clear signs of sex differentiation up to Day 90. Thus, the prolonged temporal dissociation between testicular commitment and differentiation indicates that these two events may be mediated by separate stimuli. To determine the effects of temperature on sex determination, fish of a mixed-sex population were reared at 27°C until Day 10; at 20°, 27°, or 34°C from Day 10 to Day 24; and then at 27°C until Day 102 when sex ratios and gonadal histology were determined. The sex ratio was significantly skewed toward females in fish treated at 34°C (1 male:1.68 female; P < 0.01) but no effects were seen at 20° or 27°C. This observation suggests that genetic and temperature- dependent mechanisms of sex determination are both functional in channel catfish and that this phenomenon is more widespread in fishes than previously believed. The testes of the Day 102 males examined had developed positive histological signs of sex differentiation, suggesting that testicular formation in channel catfish normally starts between Day 90 and 102

Gluttonous predators: how to estimate prey size when there are too many prey

Prey size is an important factor in food consumption. In studies of feeding ecology, prey items are usually measured individually using calipers or ocular micrometers. Among amphibians and reptiles, there are species that feed on large numbers of small prey items (e.g. ants, termites). This high intake makes it difficult to estimate prey size consumed by these animals. We addressed this problem by developing and evaluating a procedure for subsampling the stomach contents of such predators in order to estimate prey size. Specifically, we developed a protocol based on a bootstrap procedure to obtain a subsample with a precision error of at the most 5%, with a confidence level of at least 95%. This guideline should reduce the sampling effort and facilitate future studies on the feeding habits of amphibians and reptiles, and also provide a means of obtaining precise estimates of prey size

Diet and resource partitioning among anurans in irrigated rice fields in Pantanal, Brazil

Artificial ponds or irrigated systems scattered throughout farmlands can offer important habitats for anurans and can be interesting sites for research on species resources use in a changing landscape. This study describes the diet and resource partitioning among anurans inhabiting irrigated rice fields in the Pantanal region. Twenty categories of prey were found in the stomachs of Leptodactylus chaquensis, L. elenae, L. podicipinus and Rhinella bergi, the most frequent being Coleoptera, Hymenoptera, larvae of Hexapoda, Hemiptera, Diptera and Orthoptera. The great differences found in the diet of these species in rice fields compared to other locations, according to available records in the literature, was the increased importance of Hemipitera and Orthoptera and the decrease in importance of Hymenoptera in the diet of leptodactylids. These differences might be attributed to changes in the availability of resources in response to habitat modification. Although diet composition was very similar among species, niche overlap was larger than expected by chance, suggesting that the competition for food resources is not, or has not been, a significant force in determining the structure of this frog community. Two non-exclusive hypotheses could be considered as a justification for this result: 1) the high niche overlap could result from resource availability, which is sufficient to satisfy all species without any strong competition; 2) or the high values of niche overlap could be a selective force driving species to compete, but there has not been enough time to express a significant divergence in the species diet because the study area is characterised as a dynamic habitat influenced by frequent and cyclical changes