Takao Namikawa

Bio: Takao Namikawa is an academic researcher from Nagoya University. The author has contributed to research in topics: Quail & Fixed allele. The author has an hindex of 6, co-authored 7 publications receiving 492 citations.

Thyrotrophin in the pars tuberalis triggers photoperiodic response

Abstract: Molecular mechanisms regulating animal seasonal breeding in response to changing photoperiod are not well understood. Rapid induction of gene expression of thyroid-hormone-activating enzyme (type 2 deiodinase, DIO2) in the mediobasal hypothalamus (MBH) of the Japanese quail (Coturnix japonica) is the earliest event yet recorded in the photoperiodic signal transduction pathway. Here we show cascades of gene expression in the quail MBH associated with the initiation of photoinduced secretion of luteinizing hormone. We identified two waves of gene expression. The first was initiated about 14 h after dawn of the first long day and included increased thyrotrophin (TSH) beta-subunit expression in the pars tuberalis; the second occurred approximately 4 h later and included increased expression of DIO2. Intracerebroventricular (ICV) administration of TSH to short-day quail stimulated gonadal growth and expression of DIO2 which was shown to be mediated through a TSH receptor-cyclic AMP (cAMP) signalling pathway. Increased TSH in the pars tuberalis therefore seems to trigger long-day photoinduced seasonal breeding.

High genetic divergence in miniature breeds of Japanese native chickens compared to Red Junglefowl, as revealed by microsatellite analysis

Abstract: A wide diversity of domesticated chicken breeds exist due to artificial selection on the basis of human interests. Miniature variants (bantams) are eminently illustrative of the large changes from ancestral junglefowls. In this report, the genetic characterization of seven Japanese miniature chicken breeds and varieties, together with institute-kept Red Junglefowl, was conducted by means of typing 40 microsatellites located on 21 autosomes. We drew focus to genetic differentiation between the miniature chicken breeds and Red Junglefowl in particular. A total of 305 alleles were identified: 27 of these alleles (8.9%) were unique to the Red Junglefowl with high frequencies (>20%). Significantly high genetic differences (F(ST)) were obtained between Red Junglefowl and all other breeds with a range of 0.3901-0.5128. Individual clustering (constructed from combinations of the proportion of shared alleles and the neighbour-joining method) indicated high genetic divergence among breeds including Red Junglefowl. There were also individual assignments on the basis of the Bayesian and distance-based approaches. The microsatellite differences in the miniature chicken breeds compared to the presumed wild ancestor reflected the phenotypic diversity among them, indicating that each of these miniature chicken breeds is a unique gene pool.

Red jungle fowl (Gallus gallus) as a model for studying the molecular mechanism of seasonal reproduction

Abstract: Photoperiodism is an adaptation mechanism that enables animals to predict seasonal changes in the environment. Japanese quail is the best model organism for studying photoperiodism. Although the recent availability of chicken genome sequences has permitted the expansion from single gene to genome-wide transcriptional analysis in this organism, the photoperiodic response of the domestic chicken is less robust than that of the quail. Therefore, in the present study, we examined the photoperiodic response of the red jungle fowl (Gallus gallus), a predecessor of the domestic chicken, to test whether this animal could be developed as an ideal model for studying the molecular mechanisms of seasonal reproduction. When red jungle fowls were transferred from short-day- to long-day conditions, gonadal development and an increase in plasma LH concentration were observed. Furthermore, rapid induction of thyrotropin beta subunit, a master regulator of photoperiodism, was observed at 16 h after dawn on the first long day. In addition, the long-day condition induced the expression of type 2 deiodinase, the key output gene of photoperiodism. These results were consistent with the results obtained in quail and suggest that the red jungle fowl could be an ideal model animal for the genome-wide transcriptional analysis of photoperiodism.

Molecular characterization reveals genetic uniformity in experimental chicken resources.

Abstract: The objective of the present study was to conduct the genetic characterization of nine experimental chicken lines based on multilocus microsatellite analysis. Commercial chicken lines were also analyzed in order to compare their levels of genetic uniformity with those of the experimental lines. In six experimental lines, more than 80% of genotyped loci showed fixed allele for all individuals in each line, whereas only 17.5% of genotyped loci were fixed in commercial lines, at the maximum. One of experimental lines (GSN/1) was categorized as a highly inbred line on the basis of all individuals having the same, single allele at every microsatellite locus. Genetic information obtained from the present study should be helpful for the utilization and management of experimental chicken resources.

Differential development of sex-related characters of chickens from the GSP and PNP/DO inbred lines after left ovariectomy.

Abstract: To elucidate strain differences in the sex reversal of genetic females to phenotypic males, GSP and PNP/DO females were left ovariectomized (ovx) between one to three days after hatching, and the degree of masculinization based on sex-related characters, histological analysis of the right gonad and hormone assay were assessed at one year of age. The GSP and PNP/DO inbred lines were both derived from the Fayoumi breed and are only differentiated based on the red blood cell antigen type carried by each inbred line. Combs and wattles were found to be significantly bigger (P 0.05) in length compared with the respective male controls, and body weight was not significantly different (P>0.05) compared with the female controls. The weight of the right gonad was significantly heavier (P<0.05) in the GSP ovx than in the PNP/DO ovx. Positive correlations were found in the sex-related characters as well as the plasma testosterone level and right gonad weight in both the GSP and PNP/DO ovx chickens, but not in the spur length, which showed a negative correlation in the PNP/DO ovx chickens. Histological analysis revealed that the right gonads of the PNP/DO ovx chickens were morphologically developed compared with the GSP ovx chickens, which showed more advance stages of spermatogenesis. It could be inferred that PNP/DO females that exhibit a hereditary persistent right oviduct are more responsive to the masculinizing effect of ovariectomy compared with GSP females, suggesting that genetic background may have a possible contribution to the degree of masculinization and subsequent development of sex related characters.

Whole-genome resequencing reveals loci under selection during chicken domestication

Abstract: Domestic animals are excellent models for genetic studies of phenotypic evolution They have evolved genetic adaptations to a new environment, the farm, and have been subjected to strong human-driven selection leading to remarkable phenotypic changes in morphology, physiology and behaviour Identifying the genetic changes underlying these developments provides new insight into general mechanisms by which genetic variation shapes phenotypic diversity Here we describe the use of massively parallel sequencing to identify selective sweeps of favourable alleles and candidate mutations that have had a prominent role in the domestication of chickens (Gallus gallus domesticus) and their subsequent specialization into broiler (meat-producing) and layer (egg-producing) chickens We have generated 445-fold coverage of the chicken genome using pools of genomic DNA representing eight different populations of domestic chickens as well as red jungle fowl (Gallus gallus), the major wild ancestor We report more than 7,000,000 single nucleotide polymorphisms, almost 1,300 deletions and a number of putative selective sweeps One of the most striking selective sweeps found in all domestic chickens occurred at the locus for thyroid stimulating hormone receptor (TSHR), which has a pivotal role in metabolic regulation and photoperiod control of reproduction in vertebrates Several of the selective sweeps detected in broilers overlapped genes associated with growth, appetite and metabolic regulation We found little evidence that selection for loss-of-function mutations had a prominent role in chicken domestication, but we detected two deletions in coding sequences that we suggest are functionally important This study has direct application to animal breeding and enhances the importance of the domestic chicken as a model organism for biomedical research

Cellular and Molecular Basis of Deiodinase-Regulated Thyroid Hormone Signaling

Abstract: The iodothyronine deiodinases initiate or terminate thyroid hormone action and therefore are critical for the biological effects mediated by thyroid hormone. Over the years, research has focused on their role in preserving serum levels of the biologically active molecule T3 during iodine deficiency. More recently, a fascinating new role of these enzymes has been unveiled. The activating deiodinase (D2) and the inactivating deiodinase (D3) can locally increase or decrease thyroid hormone signaling in a tissue- and temporal-specific fashion, independent of changes in thyroid hormone serum concentrations. This mechanism is particularly relevant because deiodinase expression can be modulated by a wide variety of endogenous signaling molecules such as sonic hedgehog, nuclear factor-κB, growth factors, bile acids, hypoxia-inducible factor-1α, as well as a growing number of xenobiotic substances. In light of these findings, it seems clear that deiodinases play a much broader role than once thought, with great ramifications for the control of thyroid hormone signaling during vertebrate development and metamorphosis, as well as injury response, tissue repair, hypothalamic function, and energy homeostasis in adults.

Genetic diversity in farm animals--a review.

Abstract: Domestication of livestock species and a long history of migrations, selection and adaptation have created an enormous variety of breeds. Conservation of these genetic resources relies on demographic characterization, recording of production environments and effective data management. In addition, molecular genetic studies allow a comparison of genetic diversity within and across breeds and a reconstruction of the history of breeds and ancestral populations. This has been summarized for cattle, yak, water buffalo, sheep, goats, camelids, pigs, horses, and chickens. Further progress is expected to benefit from advances in molecular technology.