A cytogenetic analysis of a pericentric inversion in Hapolopappus Graclis
Haplopappus gracilis is a smaU, annual member of the family Compositae with three chromosomal races. The standard race (n=2) is distributed in the southwestern United States from southern Colorado to New Mexico and Arizona (Jackson 1973). The standard karyotype consists of a long pair of median centric chromosomes designated A and two chromosomes with submedian centromeres, secondary constrictions and satellites designated as chromosome B (Jackson 1957). A Mexican n=2 race, confined to the Big Bend area of Texas and Chihuahua and Durango, Mexico, is distinguished by a change in centromere position caused by a centric transposition on chromosome A (Jackson 1973). The third race (n=3) normally occurs in three smaU contiguous towns in Arizona. It was derived from hybrids between the n=2 standard and Mexican transposition races by chromosome breakage (Jackson 1992).
In Wickenberg, Arizona, a standard chromosome population was found to include a pericentric inversion on chromosome B. This caused a change in chromosome morphology from a normal acrocentric to a median centromere position, which was used as a marker for the inversion. The inversion includes the centromere and excludes the NOR region and satellite. Breaks caused by the inversion appear to be in nonfunctional regions because a maximum poUen fertihty of 89.45% occurs in the inversion homozygotes. Artificial crosses were performed by R. C. Jackson to obtain a homozygote inversion stock for this smdy. Photographs of the standard, inversion heterozygote, and inversion homozygote karyotypes are shown in Figures 1-3.