CLOSE KARYOLOGICAL KINSHIP BETWEEN THE REPTILIAN SUBORDER SERPENTES AND THE CLASS AVES

Gene dosage compensation and the evolution of sex chromosomes

Dosage compensation is a mechanism by means of which the activity of X-linked or Z-linked genes is made equal in the two sexes of organisms with an XX compared to XY or ZZ compared to ZW basis of sex determination. In mammals, compensation is achieved by the inactivation of one X chromosome in somatic cells of females. In Drosophila, compensation does not involve inactivation. The two X chromosomes in females as well as the single X in males are regulated, and individual genes are thought to respond independently to the regulatory mechanism. It is proposed that in both groups of organisms the evolution of heteromorphic sex chromosomes was gradual and occurred as the direct result of the evolution of dosage compensation rather than the reverse.

Karyotypic analysis and evidence of tetraploidy in the North American paddlefish, Polyodon spathula

A model chromosome number of 120 was obtained for the ancient fish. Polyodon spathula (Pisces: Chondrostei). The karyotype consists of 48 macrochromosomes and 72 microchromosomes. The microchromosomes are like those found in certain other primitive fishes as well as in reptiles and birds. The possiblity that Polyodon is a species of tetraploid origin is strongly suggested by the fact that the 120 chromosomes are easily arranged into 30 groups of four homologs each. Evolutionary comparisons are made with other primitive fish groups.

Karyotypic variation in the Australian gekko Phyllodactylus marmoratus (Gray) (Gekkonidae: Reptilia)

The gekko Phyllodactylus marmoratus has at least three distinct chromosome races; 2n=39, 2n=26 ZZ/ZW and 2n=34. Specimens from these races are morphologically distinguishable, have a degree of habitat specialization and occur in a defined distribution. The 2n=36 race found in Eastern Australia is the presumed primordial type. The 2n=34 race occurs in Western Australia and is regarded as a fusion derivative. The 2n=36 ZZ/ZW race, which is only found on the Murray River system in Eastern Australia has a heteromorphic sex chromosome system present in the female. Giemsa banding suggests that this heteromorphism is the result of a pericentric inversion.