Comparative Avian Cytogenetics: a Review

Avian cytogenetic research has, until recently, lagged far behind efforts devoted to the cytogenetics of other vertebrate groups. Avian chromosomes are inherently difficult to study because most are minute and their morphology and number are obscure. Since 1966, improved methods of culturing avian cells have resulted in more comparative chromosome studies whose quality parallels those for mammals. Recent activity in comparative avian cytogenetics now allows us to assess such factors as the overall karyotypic variability in birds and to consider the role that chromosomal change plays in avian speciation. In the present study, chromosomal variability was assessed within and among species of the same genus and within orders of birds. Chromosomal differences among local populations appear to be associated either with mechanisms that support balanced polymorphism or frequency dependent selection and not with speciation. The data are discussed in light of current models of chromosome evolution proposed for vertebrates other than birds. New cytological techniques coupled with interest in comparative cytogenetics have produced abundant data on vertebrate chromosomes, particulary those of mammals. Methods for examining the banding patterns of chromosomes and related procedures have transformed the cytogenetics of the 1960’s into a vigorous, dynamic discipline. New efforts have been made to reassess the role that chromosomal change within a lineage can play in starting reproductive isolation, and hence the speciation process. Views concerning the relevance of chromosomal change to speciation and the processes by which this might occur have polarized (Mayr 1970, White 1978). The number of avian species that have been karyotyped has doubled in the past decade (Shields 1980), yet no detailed review of this field exists and the earlier summaries of Bloom (1969) Ray-Chaudhuri (1973) and Shoffner (1974) are now incomplete. My purpose here is to synthesize all available data concerning avian cytogenetics and to interpret them in light of our understanding of chromosomal evolution in other betterstudied groups. To this extent the synthesis is comparative. Chromosomal variability between lineages at different taxonomic levels is assessed.