The Chromosome Karyotype in Estimation of Lineage Relationships

Abstract

Atchley, William R. (Dept. of Entomology, Univ. of Kansas, Lawrence, Kans. 66044)1 1972. The Chromosome Karyotype in Estimation of Lineage Relationships. Syst. Zool., 21:199-209.-The validity of phylogenetic speculations based on chromosome number and morphology alone is examined. Karyotypic attributes are examined by numerical phenetic methods in two groups of organisms where lineage relationships have been extensively studied by experimental methods. The results illustrate the inaccuracies of karyotypic data alone in inferring lineage relationships when intergenomic homologies are not available and the mechanisms of chromosomal change and direction of aneuploidy unknown. With the great proliferation of interest in chromosomal cytology by zoologists in recent years, increasing use has been made of karyotypes to infer taxonomic relationship and phylogeny. Cytological data have been used at varying levels of sophistication. Many botanists and insect geneticists have applied the truly cytogenetic approach of artificial hybridization and subsequent chromosomal analyses to gain evidence of genetic differentiation among taxa. At the other end of the spectrum, many practitioners of what might be termed have often been satisfied to simply compare different taxa on the basis of chromosome number and morphology and then speculate on genetic divergence. The use of chromosome number and morphology alone for determination of lineage relationships is suspect, since one is unable to determine chromosomal homologies in different genomes, the mechanisms involved in changes in chromosomal morphology are unknown as is the direction of aneuploidy to name some of the more outstanding problems. Further, it is intuitively obvious that in cases where the chromosomes of different genomes cannot be homologized, the degree of accuracy of the results is inversely proportional to the chromosome number. 'Present address: Dept. of Biology, Texas Tech University, Lubbock, Texas 79409. In spite of these serious shortcomings, many workers e.g., Gorman (1970), Cole (1970), Bogart (1970), Nadler (1966) and Liapunova and Vorontosov (1970) to name only a few, have used chromosome number and morphology to draw detailed conclusions about genetic divergence in various groups of vertebrates. Accurate information on homology of chromosomes in different genomes is mandatory if accurate conclusions are to be reached concerning the direction of chromosomal change. However, definitive proof of chromosomal homology, the direction of aneuploidy, etc., can generally be obtained only by experimental hybridization or, in a limited number of cases, by autoradiography. The latter technique is useful to show synchronous or asynchronous labeling patterns where translocations have occurred. Jackson (1971) has reviewed several new techniques for chromosomal analysis which show considerable promise in overcoming the above mentioned problems in organisms where hybridization is difficult or impossible. To my knowledge, however, none of these have been applied in karyotype systematics studies. The above statements and those throughout the remainder of the paper regarding the use of karyotypes relate to typical somatic chromosomes and do not necessarily hold for polytene chromosomes. In spite of the wide application of karyo-