Temporal variations in the linkages between plants and flower visitors and the pollination success of Primula modesta along the snowmelt gradient

Although pollination networks between plants and flower visitors are diverse and flexible, seed production of many plant species is restricted by pollen limitation. Obligate outcrossers often suffer from low pollinator activity or severe interspecific competition for pollinator acquisition among co-flowering species. This study focused on seasonal changes in plant–flower visitor linkages in an alpine ecosystem and examined whether and how this seasonality affected the seed-set of Primula modesta, a self-incompatible distylous herb having long-tubed flowers. First, we recorded the linkages between plants and flower visitors along the snowmelt gradient. Then, pollination experiment was conducted to estimate the degree of pollen limitation over the course of flowering season of P. modesta. Flower visitors were classified by their tongue length based on the morphological matching with P. modesta flowers. As the season progressed, plant–visitor linkages became more diverse and generalized, and the visitation frequency to P. modesta flowers increased. In the later part of the season, however, the seed set of P. modesta was significantly reduced due to severe pollen limitation, presumably because of increased competition for long-tongued pollinators among co-flowering species. The present study revealed that pollinator availability for specialist species may be restricted even when plant–visitor linkages are diverse and generalized as a whole. In the case of P. modesta, morphological matching and competition for pollinators might be the main factors explaining this discrepancy.     

Mechanism of ploidy shift from diploidy to tetraploidy in MSPC-1 mouse myeloma

In order to elucidate the cytological mechanism of ploidy shift from diploidy to tetraploidy in MSPC-1 mouse myeloma, the process of cell division was observed in living cells under phase contrast microscope. It was suggested that loss of cytokinesis and subsequent formation of binucleate cells are the major causes of such a ploidy shift. Elevated frequencies of binucleate cells during the transition phase of ploidy shift from diploidy to tetraploidy also supported the above notion. The possibility of cell fusion as a cause of the ploidy change could be eliminated by analyses of marker chromosomes and incorporation pattern of [³H]thymidine into binuclei.     

Calcium-activated chloride channel-2 in human epithelia.

Calcium-activated chloride channels (CLCAs) are a family of multifunctional proteins that are widely distributed in tissues. To investigate the distribution of human CLCA-2 (hCLCA2) in human epithelia at the light and electron microscopic levels, we raised a primary antibody against a synthetic polypeptide sequence from natural hCLCA2. Corneal, skin, vaginal, esophageal, and laryngeal epithelia were immunopositive for hCLCA2 at the cytosolic aspect of the basal cells adjacent to the basement membrane. Epithelia of stomach and small intestine showed no hCLCA2 immunoreactivity. This study reports the cellular distribution of hCLCA2 in human epithelia and suggests its possible involvement in epithelial stratification and cell-substrate adhesion.     

Rat kappa-chain allotypes

The distribution of rat kappa-chain allotype specificities (RI-1a and 1b) was studied amongRattus rattus and a variety of other Asian rodents. No sera other than those ofRattus norvegicus showed the presence of RI-1a (DA type), whereas many cross-reacted with RI-1b (LEW-type). While manyR. rattus showedtotal cross-reactivity with RI-1b, various sera from the generaRattus, Bandicota, andTokudaia showed different levels ofpartial cross-reactivity. These results indicate that (1) anti-RI-1b reagents can detectmultiple specificities on LEW-type kappa chains, and (2) these RI-1b specificities, butnot RI-1a, are widely distributed among murid rodents, in seeming contradiction to amino acid sequence data suggesting that RI-1b is more closely related to ancestral rat kappa chains.     

Novel Mouse Microsatellites: Primer Sequences and Chromosomal Location

Sixty-nine sequences containing microsatellites were determinedby analysis of clones from a pUC118 library of total genomicmouse DNA. These sequences were examined for size variationusing polymerase chain reaction and gel electrophoresis. Fifty-oneof them showed allelic variations between C57BL/6 and MSM, thetwo strains used for genetic mapping. Hence, their chromosomallocation was determined using a panel consisting of 131 backcrossmice that had been typed with 85 anchor loci. The microsatelliteswere distributed to most chromosomes except for chromosomes16 and 19. These novel markers with defined locations are usefulin linkage and genome mapping studies.     

Mauritius type black rats with peculiar karyotypes derived from robertsonian fission of small metacentrics

All seventeen black rats collected from Mauritius Island were characterized by having many extra small acrocentric autosomes. Their basic karyotype was of Oceanian type, because of the presence of the large metacentric M₁ and M₂ pairs, but chromosome numbers in 13 specimens among them were 42, those of 3 specimens 43, and those of the remaining one specimen 44. Although the Oceanian type rat had 2 small acrocentric autosomes (pair no. 13), 16 Mauritius rats had 10 small acrocentrics, and the remaining one had 8 small acrocentrics. Comparative karyotype analysis between Oceanian and Mauritius type rats showed that the extra small acrocentrics found in Mauritius rats were due to Robertsonian fission of small metacentric pairs no. 14 and 18 of the original Oceanian type rat. Only one rat with 8 small acrocentrics showed the heteromorphic pair no. 18 consisting of one metacentric and two acrocentrics. The large metacentric M₁ chromosome in 13 of 17 rats examined showed homologous pair, but two of them were heteromorphic by involving one metacentric M₁ and two acrocentrics. In the remaining two rats M₁ chromosome was not observed, but acrocentric pairs no. 4 and 7 were included. These acrocentrics were also suggested to be originated from Robertsonian fission of the large metacentric M₁ chromosome. Robertsonian fission seemed to be one of the important mechanism found in karyotype evolution.     

Sequences and evolutionary analysis of mouse 5S rDNAs

We selectively amplified the spacer regions of genes for mouse 5S ribosomal RNA (rRNA), which are tandemly repeated, by the PCR method, using primers specific to the two ends of the coding region for 5S rRNA. Fragments of approximately 1.6 kb were amplified from DNA from the BALB/cCrSlc mouse (Mus musculus domesticus), the SM/J mouse (M. m. domesticus), the MOA mouse (M. m. musculus) and the SEG mouse (M. spretus). These fragments were cloned into an appropriate plasmid vector, and two clones representative of each of the four strains were sequenced. The sequences were GC rich (> 60%) and contained a high proportion of very simple repetitive motifs, such as (TG)n and (ATCC)n, which accounted for the intra- and intergenomic length heterogeneity. Excluding such polymorphic regions and neglecting small insertions or deletions, we estimated the sequence divergence between clones. Sequence divergence within a genome averaged 0.26%, and the divergence between individuals of the same subspecies, between subspecies, and between species was 0.44%, 0.62%, and 1.73%, respectively. The results indicate that the spacer region evolved rapidly but with a reduction in heterogeneity within each genome, as a result of certain, as yet unidentified, homogenization mechanisms. The results further suggest that the spacer regions of genes for 5S rRNA may provide good indicators for phylogenetic analysis of closely related species.