The recent discovery of the DMY gene (DM domain gene on Y chromosome and one of the DMRT1 family genes) as a key determinant of male development in the medaka (Oryzias latipes) has led to its designation as the prime candidate gene for sex-determination in this species. This study focused on the sites and pattern of expression of DMY and DMRT1 genes during gonadal differentiation of medaka to further determine their roles in testis development. DMY mRNA and protein are expressed specifically in the somatic cells surrounding primordial germ cells (PGCs) in the early gonadal primordium, before morphological sex differences are seen. However, somatic cells surrounding PGCs never express DMY during the early migratory period. Expression of DMY persists in Sertoli cell lineage cells, from PGC-supporting cells to Sertoli cells, indicating that only DMY-positive cells enclose PGCs during mitotic arrest after hatching. DMRT1 is expressed in spermatogonium-supporting cells after testicular differentiation (20 -30 days after hatching), and its expression is much higher than that of DMY in mature testes. In XX sex-reversed testes, DMRT1 is expressed in the Sertoli cell lineage, similar to the expression of DMY in XY testes. These results suggest strongly that DMY regulates PGC proliferation and differentiation sex-specifically during early gonadal differentiation of XY individuals and that DMRT1 regulates spermatogonial differentiation. Developmental Dynamics 231:518 -526, 2004.
Although the sex-determining gene
SRY/Sry
has been identified in mammals, homologues and genes that have a similar function have yet to be identified in nonmammalian vertebrates. Recently,
DMY
(the
DM
-domain gene on the Y chromosome) was cloned from the sex-determining region on the Y chromosome of the teleost fish medaka (
Oryzias latipes
). DMY has been shown to be required for the normal development of male individuals. In this study, we show that a 117-kb genomic DNA fragment that carries
DMY
is able to induce testis differentiation and subsequent male development in XX (genetically female) medaka. In addition, overexpression of
DMY
cDNA under the control of the CMV promoter also caused XX sex reversal. These results demonstrate that
DMY
is sufficient for male development in medaka and suggest that the functional difference between the X and Y chromosomes in medaka is a single gene. Our data indicate that
DMY
is an additional sex-determining gene in vertebrates.
Cytochrome P450c17 is the single enzyme that mediates the 17 alpha-hydroxylase and 17, 20 lyase activities during the biosynthesis of steroid hormones in the gonads and adrenal gland. However, the mechanism underlying its dual action continues to be a controversy in the field of steroidogenesis in fish. In an attempt to resolve this issue, we identified a novel type of P450c17 (P450c17-II) by an in silico analysis from the genomes of six fish species. We cloned P450c17-II from tilapia and medaka, and comparison with the conventional P450c17-I revealed that they differ in gene structure and enzymatic activity. Enzymatic assays by thin-layer chromatography revealed that P450c17-II possesses only the 17 alpha-hydroxylase activity without any 17, 20 lyase activity, unlike P450c17-I, which has both these activities. In testis, both P450c17-I and -II express in the interstitial cells. Remarkable differences, revealed by in situ hybridization, in the expression patterns of the P450c17-I and -II in the ovary and head kidney of tilapia during various stages of development strongly suggest that P450c17-I is responsible for the synthesis of estradiol-17beta in the ovary, whereas P450c17-II is required for the production of C21 steroids such as cortisol in the head kidney. More interestingly, a temporally controlled switching is observable in the expression of these two genes during the steroidogenic shift from estradiol-17beta to the C21 steroid, 17 alpha, 20 beta-dihydroxy-4-pregnen-3-one (maturation-inducing hormone of fish oocytes) in the fish ovary, revealing a role for P450c17-II in the production of hormones that induce oocyte maturation in fish.
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