Cytogenetics and cladistics

Chromosomal data have been underutilized in phylogenetic investigations despite the obvious potential that cytogenetic studies have to reveal both structural and functional homologies among taxa. In large part this is associated with difficulties in scoring conventional and molecular cytogenetic information for phylogenetic analysis. The manner in which chromosomal data have been used by most authors in the past was often conceptionally flawed in terms of the methods and principles underpinning modern cladistics. We present herein a review of the different methods employed, examine their relative strengths, and then outline a simple approach that considers the chromosomal change as the character, and its presence or absence the character state. We test this using one simulated and several empirical data sets. Features that are unique to cytogenetic investigations, including B-chromosomes, heterochromatic additions/deletions, and the location and number of nucleolar organizer regions (NORs), as well as the weighting of chromosomal characters, are critically discussed with regard to their suitability for phylogenetic reconstruction. We conclude that each of these classes of data have inherent problems that limit their usefulness in phylogenetic analyses and in most of these instances, inclusion should be subject to rigorous appraisal that addresses the criterion of unequivocal homology.