CYTOGENETIC ANALYSES OF AUTOPOLYPLOIDS: MODELS AND METHODS FOR TRIPLOIDS TO OCTOPLOIDS

Methods are presented for determining the frequencies and numbers of various meiotic configurations expected in autopolyploids. This allows one to test polyploids of unknown origin for agreement with expected meiotic configurations. Rejection of the autoploid hypothesis may indicate the presence of Ph-like genes or some type of alloploid. The models consider mean chiasma frequencies of 2, 3, and 4 per bivalent for triploids and tetraploids and 2 per bivalent for pentaploids, hexaploids, heptaploids, and octoploids. Literature data for a known autotriploid, autotetraploids, allotetraploids, and allopentaploids were tested against expectations of the models. There was generally good agreement between number of observed autoploid meiotic configuration and those expected in the models. POLYPLOIDS are usually classified as autopolyploids, segmental allopolyploids, or true allopolyploids (Stebbins, 1947, 1950, 1970). Criteria for such a classification depend on a considerable amount of information, but a major factor is the kind of chromosome pairing in a hybrid or an individual that is known or presumed to have given rise to a polyploid. This kind of information is generally lacking in studies of natural polyploids. While it is recognized that among evolving natural populations all degrees of chromosomal differentiation may be found, the terms autopolyploid, segmental allopolyploid, and allopolyploid are semantically useful for comparative purposes even though the terms represent two extremes and the hiatus between them (Jackson, 1976). Also, one must recognize that structural changes or gene mutations affecting chromosome pairing may not affect the external phenotype (Jackson, 1971, 1976; Stebbins, 1971) so that the type of polyploid is independent of taxonomic rank. A complicating factor in polyploid classification is the presence in some populations and species of genes affecting homoeologous chromosome pairing. The best known of these is the dominant Ph gene in hexaploid and tetraploid wheats which prevents homoeologous pairing. Instead, only homologous pairing of identical chromosomes is the rule, and only bivalents are observed in normal plants (Okamoto, 1957; Sears and Okamoto, 1958; Riley and Chapman, 1 Received for publication 12 December 1980; revision accepted 17 May 1981. We thank Professor Gordon Kimber, G. Doyle, L. C. Alonso, and A. Epinasse for their helpful comments on the manuscript. 1958). When the chromosome arm or segment carrying this dominant gene is removed from a plant, intergenomal pairing occurs and multivalents are produced (Sears, 1976, 1977). Phlike but not necessarily dominant genes have been reported for diploid wheats, and Ph-like effects have been suggested for other plant species (Avivi, 1976; Sears, 1976). Evolutionists, plant breeders, and systematists working with natural polyploids generally have no quantitative methods for cytologically distinguishing between autoploids and alloploids. A major difficulty is determining the ,correct frequencies of meiotic configurations that are possible at diakinesis or metaphase I. Heretofore, there have been no definitive guidelines, but in general those polyploids with bivalents and no multivalents were considered alloploids while those with multivalents were treated as autoploids or segmental alloploids. We now know that some segmental alloploids may behave as strict alloploids due to Ph-like genes, but there has been no testable method to distinguish between autopolyploids and segmental allopolyploids in terms of multivalents. It seemed a logical construct to us that a prerequisite to classifying polyploids was the development of a method to determine expected pairing frequencies in autopolyploids. It should then follow that behavior deviant from that expected in an autoploid would have to be attributed to Ph-like genes, structural changes, or to a more gradual genome divergence which would lead eventually to the extreme of no pairing in an F1 diploid hybrid but bivalent formation in the derived, true alloploid due to what has been described as preferential pairing or differential affinity. We have developed autopolyploid models