Distribution of the 1,25 dihydroxy-vitamin D3 receptor in the bursa of Fabricius of chicken.

The vitamin D3 metabolite 1,25(OH)2D3 is probably involved in B lymphocyte ontogeny. We therefore determined the distribution of the 1,25(OH)2D3 receptor in the bursa of Fabricius and spleen cells of 7-day-old chicks, by immunohistochemistry using a monoclonal antibody against the chick intestinal cell 1,25(OH)2D3 receptor. The bursa cells of young (7-day-old) chicks contained large amounts of receptor while the spleen cells did not. The bursa cells of older (35-day-old) chicks contained fewer receptors, but the number of receptors in the spleen increased.     

Karyological conservatism in South American camelids

Summary Llama, guanaco, vicuna and alpaca show similar diploid numbers, gross chromosomal morphology and homologous G, C and NOR banding patterns. This chromosomal homology is also found in the two-humped camel and very probably in the one-humped camel as well. These findings indicate that the camelid karyotype can probably be traced back to early Miocene times. This probably represents the most extreme case of chromosomal conservatism among mammals.This work was supported by grants from the Organization of American States and SECYT.Acknowledgments. We wish to thank Dr. M. Pizarro for his assistance and encouragement.     

In vitro morphological characterization of the bursal reticuloepithelial (REp) cells of chicken.

The REp cells of the bursa follicle medulla of chicken were isolated in vitro. Culture of the REp cells was maintained over a period of 10 days and the cells were observed at 3 and 10 days by means of transmission electron microscopy (TEM) and immunofluorescence. The use of an anticytokeratin monoclonal antibody confirmed their epithelial nature. TEM observations showed the presence of desmosomes and tonofilaments, which are characteristic of epithelial cells. Furthermore, to some extent the cells regenerated in vitro the network they form in vivo. Though the growth rate becomes slower with time, the features of the REp cells do not significantly change.     

The power of VEGF (vascular endothelial growth factor) family molecules

Vascular endothelial growth factors (VEGFs) and their high-affinity tyrosine kinase VEGF receptors (VEGFRs) are key regulators of both angiogenesis and neurogenesis. The current issue of CMLS discusses recent literature and work implementing these signals in nervous system development, maintenance and disease pathology.     

Heterospecific mating between distantly related species of stink bugs and its evolutionary implications

The copulatory process in animals is preceded by a series of integrated communication processes which are often under selection to guarantee species-specific courtship and copula. However, individuals of species supposedly in reproductive isolation frequently copulate in nature, which has sparked the attention of evolutionary biologists for decades. While studies on this topic have strongly focused on recently diverged species or populations, few instances of heterospecific copula between distantly related species are known. Here we report an intriguing case of interspecific copula between two distantly related species of stink bugs in nature. We replicated this observation in laboratory conditions and revealed an asymmetric relation: males of Chinavia obstinata copulated with females of C. erythrocnemis but the reverse combination did not occur. We propose a series of hypotheses to explain our observations in the light of the current knowledge on mate recognition and reproductive isolation in stink bugs, considering the potential pre- and postcopulatory barriers involved. Once these two species have probably evolved in allopatry and contacted secondarily, the absence of directional selection against mating signals differentiation is a probable process. We argue that similarities in genital shape and size, most likely a result of convergent evolution, may have played a fundamental role in guaranteeing that these species achieved proper coupling.     

Evolutionary differentiation within the northern Great Basin pocket gopher, Thomomys townsendii II. Genetic variation and biogeographic considerations

Geographic and nongeographic variation in morphology was examined in Thomomys townsendii . A univariate analysis of external and cranial characters from a large population sample (66 adults; fusion of cranial sutures used as aging criteria) was used to assess variation among three adult age classes and between sexes. Only minor variation is apparent among age classes; however, sexual dimorphism is pronounced. Univariate and multivariate techniques were used to analyze external and cranial measurements and pelage characters for adults throughout the species range. These analyses show little to support the seven subspecific designations recognized by Davis (1937). The general pattern is one of homogeneity throughout the range of Thomomys townsendii . With the possible exception of T. t. nevadensis samples, current subspecies are not defined as morphological units. In fact, differentiation is found among populations within some subspecies. The most apparent pattern seen in these analyses is the divergence between the Humboldt River (including Honey Lake Valley samples) and Snake River systems. These results will be considered with those of a companion paper on the genetic variation in this species to more adequately assess the patterns of differentiation in Thomomys townsendii .     

Spatial and Temporal Variability in Perennial and Annual Vegetation at Chaco Canyon, New Mexico

Annual plant populations in norhtwestern New Mexico were found to be spatially and temporally highly variable. During favorable years annual plant species have patterns of dominance and diversity that are different from those of perennial species. Measurement of perennial plant diversity in plant communities is a poor predictor of productivity. Both perennial and annual components of plant communities should be considered in measurements of diversity and productivity.