Abstract
An unusual form of karyotypic variation occurs in Chironomus stigmaterus Say (Diptera: Chironomidae), a midge inhabiting the southwestern United States. Martin and Wiilker (1974) found only one individual out of 191 collected at five California localities to have an autosomal inversion. In contrast, 88 individuals from three populations in Texas and New Mexico yielded six inversions, most occurring in more than one population. Examination of collections from 11 sites in eastern New Mexico and western Texas (Hilburn, 1974) confirmed that high levels of chromosomal heterogeneity occur. In 1975 and 1976 extensive collecting was done in the southwestern United States in an attempt to characterize the distributions of inversions occurring in the species and to determine the nature of the processes which maintain the dichotomy in the inversion frequencies between the California OTU (Operational Taxonomic Unit) and the eastern OTU from Texas and New Mexico. No populations of C. stigmaterus were found between Lordsburg, New Mexico, and Blythe, California, suggesting allopatric distributions of the two chromosomally defined OTU's (Hilburn, 1979). The eight California populations examined had low levels of chromosomal heterogeneity. Only six new inversions were observed, all found in one or two heterozygotes in the Palm Springs population. In contrast, 21 inversions occurred in populations from Texas and New Mexico, with ten being found in more than one sample. Between 53% and 93% of the individuals in populations from these two states were heterozygous for at least one inversion. However, populations in the center of the study area were more polymorphic than those on the periphery. In the populations at Roswell, New Mexico, and Marathon, Texas, seven and nine inversions, respectively, were found with 93% and 90% of the animals examined being heterozygous for at least one inversion. Only two inversions occurred in the EXIT population (Fig. 1) with 53% of the sample heterozygous for at least one of these sequences. The lower level of chromosomal variability in peripheral populations makes it difficult to discern between the hypotheses that the two chromosomal OTU's represent genetically divergent forms or that they are merely ecotypes at the ends of a cline which restricts the distribution of chromosomal rearrangements without affecting the dispersal of genic mutations. The purpose of this study was to examine the genetic variability of C. stigmaterus in the southwestern United States and to obtain a more representative estimate of the genetic differences between the two chromosomally defined forms. Also, the relationship between chromosomal and genic variability in this species was examined in a preliminary way.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call