Polymorphism and evolution of influenza A virus genes

The nucleotide sequences of four genes of the influenza A virus (nonstructural protein, matrix protein, and a few subtypes of hemagglutinin and neuraminidase) are compiled for a large number of strains isolated from various locations and years, and the evolutionary relationship of the sequences is investigated. It is shown that all of these genes or subtypes are highly polymorphic and that the polymorphic sequences (alleles) are subject to rapid turnover in the population, their average age being much less than that of higher organisms. Phylogenetic analysis suggests that most polymorphic sequences within a subtype or a gene appeared during the last 80 years and that the divergence among the subtypes of hemagglutinin genes might have occurred during the last 300 years. The high degree of polymorphism in this RNA virus is caused by an extremely high rate of mutation, estimated to be 0.01/nucleotide site/year. Despite the high rate of mutation, most influenza virus genes are apparently subject to purifying selection, and the rate of nucleotide substitution is substantially lower than the mutation rate. There is considerable variation in the substitution rate among different genes, and the rate seems to be lower in nonhuman viral strains than in human strains. The difference might be responsible for the so-called freezing effect in some viral strains.      

Reconstruction of molecular phylogeny of extant hominoids from DNA sequence data

Evolutionary distance matrices of the extant hominoids are computed from DNA sequence data, and hominoid DNA phylogenies are reconstructed by applying the neighbor-joining method to these distance matrices. The chimpanzee is clustered with the human in most of the phylogenetic trees thus obtained. The proportion of the distance between human and chimpanzee to that between human/chimpanzee and orangutan is estimated. Both mitochondrial DNA and nuclear DNA show a similar value (0.44), which is close to values derived from DNA-DNA hybridization data.     

Floaters as intraspecific brood parasites in the grey starling Sturnus cineraceus

The breeding season of the grey starling Sturnus cineraceus is divided into two periods: the early and late breeding season. The birds that breed in each season are referred to as early and late breeders, respectively. In this study, the late breeders mainly consisted of new immigrants that did not breed in the early season. This suggested that these new immigrants were probably floaters in the early breeding season. Because intraspecific brood parasitism occurred frequently, it is possible that the parasites were floaters without nesting boxes. To check for the presence of floaters, two field experiments were conducted and floaters were captured with traps. With additional nesting boxes provided during the breeding season, all new boxes were quickly occupied by floaters from the period of incubation to hatching in the early breeding season, but were not occupied by floaters during the egg laying period of the late breeding season. The addition of boxes before the start of the breeding season significantly decreased the parasitic rate and number of parasitic eggs per nest. There was a positive correlation between the relative occupancy of nesting boxes and the parasitic rate. The removal of boxes again increased the parasitic rate. As for the capture of floaters with traps, the number of trapped birds per day was also related to the relative occupancy of nesting boxes. The floaters trapped between incubation and nesting periods of early breeders became the late breeders. Judging from these results, many floaters were present from the incubation to hatching periods of early breeders, and were probably intraspecific brood parasites.     

Unravelling of physiological functions of retinoids using a dominant-negative retinoic acid receptor

We have constructed dominant-negative retinoic acid receptors by substituting a single amino acid which has been found in a dominant-negative thyroid hormone receptor, and have expressed the dominant-negative retinoic acid receptors in the epidermis, a potential target organ of retinoic acid. The resultant transgenic mice exhibited dramatic suppression of epidermal development, demonstrating the absolute requirement of retionic acid in normal skin development. This novel method, targeted expression of the dominant-negative receptor, is theoretically applicable to any organ, thus opening the way to defining the physiological roles of retionic acid as well as other lipophilic hormones during embryogenesis as well as in adults