A cytogenetic and electrophoretic investigation of selected diploid and tetraploid populations of Haplopappus gooddingii

Abstract

To address the evolutionary status of the hypothesized natural autopolyploids of Haplopappus qooddingii (Compositae), this study focused on quantifying meiotic chromosome pairing, assessing levels of genetic variation within and among populations, and evaluating the genetic relatedness of the diploid and polyploidy levels. Autopolyploidy results from the doubling of a diploid chromosome set with homozygous pairing control genes, and allopolyploidy involves chromosomal doubling of two different chromosome sets that are heterozygous for pairing control genes. Meiotic pairing in a chromosomally doubled diploid is determined by the pairing control gene condition. If the hybrids are heterozygous, the derived tetraploids will differ significantly from the expected autoploid meiotic configuration frequencies, and strict allopolyploidy will exhibit only bivalent pairing and a lack of quadrivalents. Data from meiotic pairing configurations and isozyme analyses were obtained from both ploidy levels. The diploids exhibited normal nonrandom meiotic pairing, but the tetraploids failed to conform to autoploid expectations. The diploids had high pairwise genetic identities (I>0.894) that are consistent with conspecific populations. Vcdues of genetic identity were equally high among the tetraploids, and when these are integrated with diploid pairwise comparisons, values are slightly lower (I>0.857). UPGMA clustering failed to separate the diploids from the tetraploids and embedded the tetraploids within a cluster containing a subset of diploid populations. This is consistent with a putative intraspecific origin for the tetraploids. These data suggest that the tetraploids may have begun as autopolyploids, and as a result of pairing control mutations have drifted or undergone selection for alloploid chromosome behavior. This would explain the lower than expected frequencies of quadrivalents with a corresponding increase in bivalents during meiosis, and this would be consistent also with the isozyme evidence.