It is well known that the sex in mammals is determined by the X and Y chromosomes, the XY being male and the XX being female, whereas the sex in birds is determined by the Z and W chromosomes (ZZ males and ZW females). These XY and ZW systems have a history of about 200 million years. Therefore, the sex determination system seems to be quite stable once it is established (1).
In amphibians, however, this is not the case, and the same taxonomic family may include species with both the XY and ZW systems (2). In an extreme case the same species contains both the XY and ZW systems as polymorphism. This situation is observed in the Japanese frog Rana rugosa. This species inhabits almost the entire territory of Japan, but depending on the geographical area the chromosomal system is either the XY or the ZW (see Fig. 1). The ZW system is heterogametic, and therefore the male and female can be distinguished by the chromosomal morphology. However, the XY system is either heterogametic (Hamakita areas) or homogametic (Hiroshima and Isehara areas) depending on the geographical area (3). Further studies have shown that the gene sequence arrangements of the X and Y chromosomes from Hiroshima and Isehara are different from each other (Fig. 1). How did these chromosomal systems evolve? Solution of this question may allow us to understand the general scheme of evolution of sex determination.
Fig. 2. A phylogenetic tree of the ADP/ATP translocase gene from the six kinds of sex-determining chromosome 7. The three kinds of chromosome 7 are shown on the right: S = subtelocentric; mS = more subtelocentric; M = metacentric.
Miura et al. (3) studied the evolutionary history of these sex chromosomes by using the sex-linked gene, AP/ATP translocase gene, on the sex chromosome (chromosome 7). The phylogenetic tree constructed from this gene is presented in Fig. 2. On the basis of this phylogenetic tree, they proposed the evolutionary scenario presented in Fig. 3. This scenario suggests that the “subtelocentric chromosomes” in the Hiroshima area first generated the “more subtelocentric chromosomes” similar to the chromosomes from the Isehara area. The parental “subtelocentric chromosomes” are then hybridized with the “more subtelocentric chromosomes” to produce the “metacentric chromosomes.” Finally, these “metacentric chromosomes” and the Hiroshima “subtelocentric chromosomes” generated the XY and the ZW chromosomal systems. These evolutionary changes are believed to have occurred relatively recently.
Fig. 3. A phylogenetic pathway of the sex-determining chromosome 7 inferred from the ADP/ATP translocase gene tree. The subtelocentric chromosome is shown by a white area, the more subtelocentric by shaded area, and the metacentric by black area.
However, there are several problems in this scenario. The first and most important one, which the authors also recognized, is the nature and the location of sex-determining genes on the sex chromosomes. The sex is determined not by chromosomes but by genes, and therefore it is essential to identify the genes that trigger the formation of testis or ovary. Chromosomal morphology can change relatively easily by inversion and translocation, but the sex-determining genes are generally more conserved, and therefore they are likely to give a more accurate pattern of evolution of sex determination. Furthermore, the morphological study of chromosomes does not give information on small gene deletions, insertions, or pseudonization. It is therefore difficult to reconstruct the evolutionary history of sex determination in the present case.
For the above reason, the authors have not clearly distinguished between the X and Y chromosomes in the Hiroshima and Isehara areas (Fig. 1 and 3). Fig. 2, however, indicates that the X and W chromosomes and the Y and Z chromosomes are evolutionarily related, respectively. Although the phylogenetic tree was constructed by using a sex-linked gene rather than the sex determining gene, the results presented are interesting if we consider the mammalian sex determining gene, SRY, that initiates the pathway of testis formation. In this case the individual without the SRY gene becomes female as a default option. In birds, which have the ZW system, the male (ZZ) is believed to form testis because of the presence of two copies of the sex determining gene, Dmrt1, and the ZW female produces ovary as a default option (2, 4).
At the present time we know nothing about the molecular basis of sex determination in frogs. However, if we use the sex determination in mammals and birds as guidance, the evolutionary changes of the XY and ZW systems may be studied at the molecular level.
In this connection, Miura et al. (5) presented an interesting observation with respect to the accumulation of lethal mutations in the Y and W chromosomes. In frogs it is possible to generate the WW homozygotes by producing the ZW males with a hormone treatment and by crossing the ZW males with the normal ZW females (5). This WW homozygote did not survive well in the tadpole stage, and most of the individuals died sooner or later. When the WW homozygotes were produced from various geographical locations, it was clear that the W chromosome carries a lethal mutation but the lethal mutation varies from location to location. In other words, the W chromosome evolved independently in different locations. Therefore, when the WW was produced by using the W chromosome from different locations, the WW homozygotes could survive up to adulthood.
The same type of experiment was done with the Y chromosome. In this case the YY individual could not survive even if the two different Y’s were derived from different locations. This suggests that the XY system evolved earlier than the ZW system.
It has been argued that once a set of linked genes related to sex determination evolve in a chromosome the recombination value of the linked gene region tends to be reduced by modifier genes (6) or chromosomal inversions (7) and the recessive lethal mutations gradually accumulate in the region (8, 9). The rate of accumulation of lethal mutations depends on the mutation rate, population size, and evolutionary time.
The evolutionary change between the XY and ZW systems also occurs more frequently in small populations than in large populations. Because the R. rugosa frogs generally inhabit the mountainous areas, the effective population size must be very small. Probably for these reasons, R. rugosa have undergone rather rapid changes of the sex chromosome (chromosome 7) and the genes controlling sex determination. However, the evolutionary changes of sex determining genes are poorly understood at present. It is hoped that a more detailed molecular study will be conducted for understanding the evolutionary mechanism of sex determination in the future.
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