Evolution of sex chromosomes and heterogametic systems in amphibia

Evidence for heteromorphic sex chromosomes in males of Rana tagoi and Rana sakuraii in Nishitama district of Tokyo (Anura: Ranidae)

Karyotypes of the Tago brown frog Rana tagoi and stream Tago brown frog Rana sakuraii from a mountain region in the Nishitama district in Tokyo were examined by conventional Giemsa staining, C-banding and late replication (LR)-banding. Chromosome number was 2n = 26 in all cases. The 26 chromosomes consisted of five (1-5) pairs of large chromosomes and eight (6-13) pairs of small chromosomes. Chromosome 10 had a secondary constriction on the long arm. In all frogs, on chromosome pair 8, the XX/XY type sex chromosome was present. C-banding analysis indicated that, in R. sakuraii, neither the X nor Y chromosome possessed interstitial C-bands but each had centromere staining, while in R. tagoi, an interstitial C-band was present on the long arm of the X chromosome. The Y chromosome had no interstitial C-band. LR-banding analysis demonstrated the X and Y chromosomes to have a LR-band on the short arm and two LR-bands, each on the long arm, and the bands on both species to be essentially the same. Heteromorphic sex chromosomes in males of R. sakuraii and R. tagoi were identified for the first time in this study.

Chromosome banding in Amphibia. XVII. First demonstration of multiple sex chromosomes in amphibians: Eleutherodactylus maussi (Anura, leptodactylidae)

A cytogenetic study performed on a population of the South American leptodactylid frog Eleutherodactylus maussi revealed multiple sex chromosomes of the X1X1X2X2 female/X1X2Y male (= XXAA female/XXAY male) type. The diploid chromosome number is 2n = 36 in all females and 2n = 35 in most males. The multiple sex chromosomes originated by a centric fusion between the original Y chromosome and a large autosome. In male meiosis the X1X2Y (= XXAY) multiple sex chromosomes form a classical trivalent configuration. E. maussi is the first species discovered in the class Amphibia that is distinguished by a system of multiple sex chromosomes. Only one single male was found in the population with 2n = 36 chromosomes and lacking the Y-autosomal fusion. This karyotype (XYAA male) is interpreted as the ancestral condition, preceding the occurrence of the Y-autosome fusion.

Chromosome banding in Amphibia. XII. Restriction endonuclease banding

Fixed metaphase chromosomes of several species of Amphibia were treated with various restriction endonucleases and subsequently stained with Giemsa. Metaphases of man and chicken were examined in parallel under the same experimental conditions for comparison. The restriction enzymes always induce subsets of the C-banding patterns present in the amphibian karyotypes. The heterochromatic regions can be either resistant or sensitive to the restriction enzyme. The modified C-banding patterns revealed by different restriction endonucleases in the karyotype of the same species can be either extremely dissimilar or almost completely congruent. Correspondingly, the action of the same restriction enzyme on the karyotypes of different species may vary greatly. There is only rarely a correlation between the type of C-banding patterns produced by different restriction endonucleases and their specific base pair recognition sequences. In contrast to mammalian and avian chromosomes, restriction enzymes induce no multiple G-banding patterns in amphibian chromosomes. This is attributed to the difference in organization of the DNA in the genomes of poikilothermic vertebrates. The possible mechanisms of restriction endonuclease banding and the various uses of this technique for amphibian chromosomes are discussed.

Sex determining mechanisms: an evolutionary perspective

Theories on the evolution of sex determining mechanisms are reviewed for male and female heterogamety, environmental sex determination, and briefly, haplo-diploidy and hermaphroditism. Because of their discrete and well-defined nature, sex determining mechanisms lend themselves to three types of evolutionary questions: what variety occurs and might be expected but does not occur, how do changes occur from one mechanism to another, and why do certain changes occur? All three approaches were illustrated for these different sex determining mechanisms. A generality emerging from these studies is that, at the level of selection of the sex ratio, there are no intrinsic problems in evolving from one sex determining mechanism to another: straightforward transitions between different mechanisms exist under various conditions.