Abstract
Chromosomal and isozyme variation was examined in populations of the ant Aphaenogaster ‘rudis’. Georgian populations of this ant are referable to a lighter Coastal Plains or a darker Montane phenotype. Coastal Plains populations are fixed or nearly fixed for a null esterase allele, Es 0, whereas high activity allele frequencies characterise Montane populations. High Mdh-a 1 frequencies (>0.5) are typical of Coastal Plains populations, but in Montane populations this allele is rarer (<0.3) or absent. Study of a narrow (approximately one kilometer wide) contact zone (Turner's Corner) between these two forms showed that Coastal Plains ants have n=20 as against n=22 for Montane ants there, that there was no chromosomal evidence of hybridization, but that the presence of low frequencies of the alternate Es alleles in both forms indicates that there is some introgression. The Montane phenotype is itself heterogeneous at Black Rock Mountain, where a further, 18-chromosome, cytotype shows marked microhabitat segregation from the sympatric 22-chromosome form and differs from it in allele frequencies at all four loci examined (Mdh-a, Mdh-b, Es, Amy). Average relative genetic distances (Nei's Dm) between populations within all forms are low (maximum of 0.03±0.01) compared with interform distances (minimum of 0.19±0.03), which are similar to distances between the ‘rudis’ forms and both of fulva and treatae. Aphaenogaster fulva has a very similar karyotype to that of 18-chromosome ‘rudis’, in which some specimens show weak development of the taxonomic characters distinguishing fulva from ‘rudis’. However, fulva and 18-chromosome rudis differ markedly allozymically, especially in that Amy 5 and Amy 6 are the only amylase alleles in fulva and Amy 4 is the only amylase allele in 18-chromosome ‘rudis’. An Alachua County, Florida rudis-group population has a 22-chromosome karyotype different to that of Georgian ‘rudis’, but the genetic distance values suggest it may be most similar genetically to the Montane 22-chromosome form. Low-level gene flow between fulva and rudis-group populations is suggested by similarity of fulva and two different sympatric rudis-group populations in terms of Es alleles present.
The Georgian 20- and 22-chromosome ‘rudis’ karyotypes may possibly be related by Robertsonian changes, but the relationship between these and the 18-chromosome forms (18-chromosome ‘rudis’, fulva, and an 18-chromosome Florida isolate), treatae (n=21), and lamellidens (n=19), are obscure. A non-Robertsonian chromosome number polymorphism was found in one Coastal Plains population.
No unequivocal evidence was found for an overall departure of genotype frequencies from those expected under the Hardy-Weinberg Law.
The ‘rudis’ cytotypes in Georgia are sibling species whose close morphological resemblance may reflect ecological but not genetic similarity.
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Crozier, R.H. Genetic differentiation between populations of the ant Aphaenogaster ‘rudis’ in the Southeastern United States. Genetica 47, 17–36 (1977). https://doi.org/10.1007/BF00122435
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DOI: https://doi.org/10.1007/BF00122435