This study was designed to explore the relationship of estrogen, human chorionic gonadotropin (HCG), and food availability to endocytosis in developing oocytes. When estrogen alone is administered to an animal, large amounts of vitellogenin are synthesized by the liver and secreted into the circulatory system, where it accumulates. Under these conditions there is no evidence of endocytosis at the surface of the oocytes. Other studies have shown that following HCG injection into estrogen-treated animals, vitellogenin is removed from the circulation and the oocyte surface is highly contoured and displays endocytotic activity. Food deprivation has much the same effect on oocyte endocytosis as does estrogen. When animals are given HCG and subsequently starved for 20 days, developing oocytes show little endocytotic activity. We conclude that HCG acts to promote or stimulate endocytosis in developing oocytes while estrogen and/or starvation inhibits this process.

Oogenesis in Xenopus laevis (Daudin)

The migratory and reproductive behaviour ofBufo calamita was studied at four neighbouring breeding sites in the northern Rhineland, Germany, from 1986 to 1991. Radio telemetry and marking systems based on toe-clipping and on microchips were used to follow the tracks of toads and for individual recognition. Emphasis lay on estimates of (1) the exchange of reproductive individuals between neighbouring sites, and (2) the reproductive success at each site. Allozyme electrophoresis served to assess the genetic diversity of local populations. More than 90% of all reproductive males showed a lifelong fidelity to the site of first breeding, whereas females did not prefer certain breeding sites. Due to the female-biased exchange of individuals among neighbouring sites the genetic distance between local populations was generally low but increased with geographical distance. This pattern of spatial relations is consistent with the structure of a metapopulation. Morever, up to three mass immigrations of males per breeding period, replacing previously reproductive individuals, suggested the existence of temporal populations successively reproducing at the same locality. Genetic distances were considerably greater between temporal populations than between local ones, indicating partial reproductive isolation. In fact, an exchange of reproductive individuals between the temporal populations at each site was not detected, but gene flow due to the recruitment of first-breeders originating from offspring other than their own seems probable. Thus, natterjack metapopulations consist of interacting local and temporal populations. The reproductive success differed considerably among the four sites and also between the temporal populations. Three out of four local populations had low reproductive success as well as the latest temporal population. The persistence of these populations depended entirely on the recruitment of juveniles from the only self-sustaining local population. This “rescue-effect” impeded local extinction. The discussion focuses on the modifications required to fit the classical metapopulation concept to the empirical findings and their consequences for the dynamics of amphibian metapopulations.

Structure and dynamic of a natterjack toad metapopulation (Bufo calamita).

We censused pond-breeding hylid frogs in northeastern Costa Rica weekly for 15 months to deseribe patterns of reproduction, habitat use, and to establish baseline data on relative abundance for members of the assemblage. Reproduction in the Costa Rican assemblage was seasonal and occurred only during wet months. Some species called, but none reproduced, during the dry season. Three species (Agalychnis callidryas, Hyla ebraccata, and Scinax elaeochroa) accounted for more than 75% of the observations made during the study. The species overlapped broadly in time and space, but differed in substrate use and phenology. Two species of leaf-breeding frogs (A. callidryas and A. saltator) used perches that were significantly higher than those used by the other species. Some phenological differences were associated with different mating strategies. Explosive breeders (Scinax elaeochroa and Smilisca baudinii) were most common early in the wet seasons. Prolonged breeders (A. callidryas and H. ebraccata) were the most persistent members of this assemblage. Predation affects early and late life history stages of these hylids. Predation on arboreal egg masses by two snake species was observed. Ctenid spiders preyed on recently metamorphosed frogs and small adults. Our weekly samples were pooled into 21-day periods so that we could compare our results with those obtained for two communities of breeding anurans from South America. The patterns observed in the Costa Rican assemblage differed from those reported for South American pond-breeding frogs, but in all three assemblages reproduction was associated with wet periods.

Patterns of reproduction and habitat use in an assemblage of Neotropical hylid frogs.

Although anuran amphibians are diverse and conspicuous in many vertebrate communities, worldwide population declines have been observed. Climatic change is a global factor that has been implicated in some of these declines. In this paper, we speculate on how Neotropical anurans might respond to changes in climate predicted by Hulme and Viner (1998). We focus on two distinct groups of Neotropical anurans: frogs that live and oviposit in leaf litter and frogs that congregate at ponds to breed. Increased temperature, increased length of dry season, decreased soil moisture, and increased inter-annual rainfall variability will affect Neotropical frogs strongly. We expect that these changes will directly affect frogs by changing reproductive success and breeding periodicity, and indirectly by altering the invertebrate prey base. The individual effects will likely translate into changes at the population and community levels. We also speculate on how climatic change will affect Neotropical amphibians that are restricted ecologically and/or geographically. We suggest directions for future research that will increase our ability to predict how amphibians in the New World tropics will respond to climatic change.

Potential Effects of Climate Change on Two Neotropical Amphibian Assemblages

synopsis. The ultimate control of amphibian gonadal function rests with environmen? tal factors mediated through the hypothalamus. It appears that control of ovarian growth resides in the infundibular region and ovulation in the preoptic area. For normal temporal relationships between oocyte growth and ovulation to occur, an intact hypothalamo-pituitary complex is necessary. It is uncertain whether the several types of pituitary basophils considered histologically as gonadotropin-producing cells are in fact producing separate LH- and FSH-like hormones. Perhaps the concensus indicates a single hormone has both vitellogenic and ovulatory functions. This hormone stimulates estrogen synthesis and secretion by the ovarian follicle cells, and this steroid causes oviduct growth and the hepatic biosynthesis of vitellogenin, the major yolk platelet precursor. Uptake of this lipoprotein from ihe circulation and its conversion to the components of the platelet is mediated by the gonadotropin, the presence of which results in the estaiblishment of a rapid micropinocytotic process at the level of the oocyte surface and of a mechanism for crystallization of the yolk. A sudden surge of pituitary hormone, when presented to fully grown oocytes leads to their maturation and ovulation, and to oviducal jelly release in some species. The active hormone is progestin in nature, again produced by the follicle cells. In this review the known factors involved in the hypothalamo-hypophysio-ovarian axis are discussed together with some consideration of outstanding problems and the possible relevance of oviparity and ovoviviparity in amphibians to the ovarian control found in viviparous species. Both the gametogenic and endocrine functions of the amphibian ovary are un? der pituitary control, but as in other lower vertebrate classes, the precise nature of this control, and the overriding regulation of ovarian cycles by the hypothalamus and central nervous system, leaves large areas where knowledge falls short of that for mammals. The fundamental features of reproduction in the female seem remark? ably uniform throughout the vertebrates although recourse to other contributions to this symposium will indicate inter-class differences in the emphasis placed on the various hormonal factors. Perhaps t'he most significant of these differences is the role of the estrogenic steroids in mediating the process of vitellogenesis. This process is, of course, of paramount importance in all oviparous species, and the gonadotropins fulfill their function of controlling ovarian growth by regulating estrogen synthesis

The Hormonal Control of the Amphibian Ovary

In the Bangalore population of Bufo melanostictus the ovarian cycles in individual toads are characterized by synchronous growth of the vittellogenic oocytes and by asynchrony with respect to stage in the ovarian cycle among the toads. However, late ovarian cycles predominated during the period of August-October. The incidence of atresia, as observed in the ovaries sampled throughout the year, suggests that about two thirds of the toads preparing for spawning are primiparous, leaving only one third to represent iteroparity within the population. Body size varied greatly at first maturation, when somatic growth declined or stopped. The greatly varying body size is consistent with a seasonal timing of sexual maturation preparing for the main breeding season, which coincides with the height of the monsoon rains during August-October.

https://www.jstor.org/stable/info/3565838

Body size, reproduction and growth in a tropical toad, Bufo melanostictus, with a comparison of ovarian cycles in tropical and temperate zone anurans

Regulation of ovarian cycle in the toad Bufo bufo bufo (L.): effects of autografting pars distalis of the hypophysis, of extirpating gonadotropic hypothalamic region, and of partial ovariectomy.

S. Vijayakumar

Papers

Body size, reproduction and growth in a tropical toad, Bufo melanostictus, with a comparison of ovarian cycles in tropical and temperate zone anurans

Regulation of ovarian cycle in the toad Bufo bufo bufo (L.): effects of autografting pars distalis of the hypophysis, of extirpating gonadotropic hypothalamic region, and of partial ovariectomy.

Annual oviduct cycle and its control in the toad, Bufo bufo L.

Is the toad pars intermedia under both excitatory and inhibitory nervous control