Genetic Regulation of Melanocyte Differentiation

Melanocytes originate from the neural crest in vertebrates and migrate to the body surface where they differentiate into functional cells. Genes involved in melanocyte differentiation can be classified into two groups. One of them consists of the functional genes that control proteins specific to the function of the melanocyte. As the representative gene of this category, albino (c) locus in the mouse is considered to control tyrosinase, the key enzyme in melanogenesis. cDNA for mouse tyrosinase has been cloned and sequenced. The cDNA can be used to detect tyrosinase mRNA synthesized during melanocyte differentiation. On the other hand, genes such as brown (b) or pink-eyed dilution (p) have been assumed to control melanosome proteins. The other category consists of genes that regulate the expression of these functional genes directly or indirectly. In the mouse, so-called white-spotting genes and genes of the agouti series are considered to fall into this category. Based on the fact that mutations at the white-spotting loci result in the absence of melanocytes in a particular area of skin, it is assumed that some of these loci control the factors that promote either differentiation or migration of melanoblasts and are candidates for the classic regulator genes Genes at the agouti (a) locus in the mouse determine the type of melanin synthesized in hair follicle melanocytes, that is eumelanin or pheomelanin. An interesting feature of this locus is that the site of gene action is not within the melanocytes but in the cells surrounding them. The results of our study indicate that the gene product of the a-locus interacts with α-MSH at the α-MSH receptor site, regulates the cellular cAMP level via a signal transduction system and, in turn, determines the type of melanin synthesized in the cells.     

Differentiation of Extracutaneous Melanocytes in Embryos of the Turtle,Trionyx sinensis japonicus

The present study investigates the mode of differentiation of neural crest-derived melanocytes in the embryos of the soft-shell turtle, Trionyx sinensis japonicus. DOPA reaction-positive melanoblasts were first detected in 10-day-old embryos. Melanocyte differentiation in terms of pigmentation takes place from the day 16 of development. Melanin pigments were found in the dorsal integument as well as in various extracutaneous tissues such as skeletal muscle, dorsal aorta, peritoneum, blood vessels, choroid, lung, bone marrow, fat tissues and in the connective tissue of the nose. These results suggest the presence of a specific environmental regulation of the melanoblast differentiation in the soft-shell turtle.     

Melanocytes Fail to Survive in Hair Bulbs of the Shiba Goat (Capra hircus) With the Dominant Black-Eyed White Phenotype

Dominant black-eyed white phenotypes are one of the most commonly observed traits in domestic animals. Their genetic control mechanisms, however, have not been elucidated. As the first step to approach the problem, we examined histologically the patterns of the distribution of pigment cells in Shiba goats (two each of day-73-postcoitum and day-112-postcoitum fetuses, and a 15-week-old kid) with the dominant black-eyed white phenotype. Melanocytes were present and fully pigmented in the choroid and the sclera of eyes, as well as in dorsal skin epidermis of the fetuses and of the kid. Melanocytes were also found in approximately 6% of the hair bulbs in the fetal dorsal skin, while the rest (94%) lacked them. Hair follicles of the kid did not harbor melanocytes except for some in the early anagen stage. The results suggest that the survival of melanocytes was inhibited specifically in the hair follicles of the Shiba goat with the dominant black-eyed white phenotype and that the ostensibly similar phenotypes in the Shiba goat and in the SI or W mutants of the mouse, where melanocytes die en route to the hair bulbs, are regulated by different mechanisms.     

Phylogenetic relationships of subgenera of the genus Exorista Meigen,with a revision of the Japanese species (Diptera: Tachinidae)

Phylogenetic relationships of the subgenera of Exorista Meigen (Diptera: Tachinidae) are inferred from morphological data. Our results show that the genus Exorista is not monophyletic and that members of the subgenus Spixomyia Crosskey are divided into two clades. Each subgenus is redefined based on male and female morphological features. The Japanese species of Exorista are revised and classified into five subgenera: Adenia Robineau‐Desvoidy, Exorista Meigen, Podotachina Brauer and Bergenstamm, Ptilotachina Brauer and Bergenstamm, and Spixomyia Crosskey. Thirteen species are recognized, including two newly recorded species, Exorista (Adenia) cuneata Herting and Exorista (Spixomyia) lepis Chao. Exorista cantans Mesnil is transferred to the subgenus Podotachina from Spixomyia.     

Expression of Tyrosinase Gene in Transgenic Albino Mice: The Heritable Patterned Coat Colors

To elucidate the regulatory mechanism for tyrosinase gene expression in vivo, we microinjected a mouse tyrosinase minigene, mg-Qrs-J, into the fertilized eggs of BALB/c albino mice. As a result, we obtained six pigmented founder mice that exhibited non-standard coat color variations as well as the wild-type phenotype. These founder mice were subsequently crossed with BALB/c albino mice to establish the transgenic lines. As a consequence, two primary lines and five sublines have been obtained from four of the six founder mice. We found that not only uniformly pigmented phenotypes but also patterned phenotypes were inherited by their descendants. The possible underlying mechanism of the patterned phenotypes is discussed.     

Quantitative Analysis of Striped Coat-Color Patterns in Large White→Duroc Chimeric Pigs With Special Reference to the Genetic Control Mechanisms of the Dominant Black-Eyed White Phenotype

Coat colors of four chimeric pigs produced by the microinjection of dissociated blastomeres of (Landrace × Large White) blastocysts to the blastocyst cavity of (Duroc × Duroc) blastocysts (Kashiwazaki et al., 1992) exhibited characteristic horizontal stripe-patterns. We carried out quantitative analysis of those patterns in order to derive information concerning the genetic regulatory mechanisms of the dominant black-eyed white phenotypes in the pig. In the four chimeras, the theoretical mean widths of the single-clone stripe calculated from the estimated widths of minimal recognizable stripe (MRS) (Tachi, 1988) were 2.1 ± 0.1, 2.23 ± 0.15,1.89 ± 0.06, and 1.93 ± 0.28 cm respectively. The estimated number of single-clone stripes in the thoracico-lumbar region of those animals were 42.3, 40.7, 46.3, 44.2, and about twice the mean number of vertebrae in the same region (Duroc, 20 or 21; Large White, 21 or 22). Furthermore, the mean length of thoracico-lumbar vertebrate in two of the chimeric pigs, as measured on X-ray radiographs, was approximately twice the mean single-clone stripe width. It was concluded that the stripe-patterns of the chimeric pigs probably represented the dermatome patterns of epidermis; and in the pig, a single somite was likely to be derived from the clones of two primordial cells, as originally proposed by Gearhart & Mintz (1972) in the mouse. It was suggested, furthermore, that in the Large White→Duroc chimeric pigs, melanocytes that migrated into the region of skin formed by a Large White dermatome could not survive, thus creating a clearly demarcated white stripe. Possible involvement of KL or c-kit in the dominant black-eyed white phenotype of the pig is discussed.     

Molecular phylogeny of the subfamily Exoristinae (Diptera,Tachinidae), with discussions on the evolutionary history of female oviposition strategy

Abstract. Phylogenetic relationships among tribes in the tachinid subfamily Exoristinae (Diptera, Tachinidae) are inferred from four genes, namely white, 18S, 28S and 16S rDNA. For phylogenetic inferences, maximum parsimony, maximum likelihood and Bayesian Markov chain Monte Carlo analyses were performed. The resultant, very similar, trees are nearly concordant with the traditional classification based on morphological characters. Our results suggest that the Tachinidae are monophyletic and sister to the Sarcophagidae. The tribal relationships within Exoristinae are supported in part with high reliabilities and are similar to those inferred by Stireman. Based on the resultant trees, the phylogenetic relationships and possible morphological synapomorphies were investigated. In addition, we evaluated the transformation of female reproductive habits in the Exoristinae, finding support for the hypothesis that ovolarviparity evolved independently from oviparity in several clades, and obtaining different results concerning the evolutionary history of micro‐ovolarviparity depending on character optimization.     

Revision of the subgenus Ceranthia Robineau-Desvoidy of the genus Siphona Meigen of Japan (Diptera: Tachinidae)

Japanese species of the subgenus Ceranthia Robineau‐Desvoidy are revised. Five species are recognized in Japan, three of which are described as new to science: Siphona (Ceranthia) angusta, S. (C.) nigra and S. (C.) setigera. A key to Japanese species is provided and male terminalia are illustrated. Monophyly of this group is discussed.