Organisms usually have two sex chromosomes.In rare cases, a sex chromosome and another chromosome (autosomal chromosome) fuse to form a complex sex chromosome consisting of multiple sex chromosomes, but the evolutionary reason is unknown. Forty-one years ago, Dr. Mitsuru Kuramoto of the University of Teacher Education Fukuoka investigated the Sinhohanasaki frog inhabiting Taiwan and discovered the complex sex chromosome for the first time in a frog.Therefore, this time, we investigated the chromosome of this frog in detail using a new analysis method.

　As a result, it was found that the male had 3 X chromosomes and 3 Y chromosomes, and the female had 6 X chromosomes, and that it was born by fusing 3 chromosomes in a three-way manner.Furthermore, it was found that one of the three has sex-determining genes for eutheria such as birds and platypus (and fish), and the other for humans.

　In the case of frogs, there are at least 6 types of sex chromosomes, and these 6 are called latent sex chromosomes.Normally, when one function as a sex chromosome, the other five are reserved as autosomal chromosomes, but as crossbreeding between populations and genetic differentiation of new populations progress, another chromosome switches to a sex chromosome.This is called the "replacement (turnover)" of the sex chromosome.

　In this research result, all the fused chromosomes were latent sex chromosomes.Therefore, it is possible that the selection of fusion chromosomes was not random but inevitable.This is a new interpretation of the evolutionary reason for the birth of complex chromosomes, and is said to be a clue to elucidate the evolution (and replacement) mechanism of sex chromosomes.

Paper information:[Cells] Evolution of a Multiple Sex-Chromosome System by Three-Sequential Translocations among Potential Sex-Chromosomes in the Taiwanese Frog Odorrana swinhoana