Ecological genetics of monogynous and polygynous forms of the red imported fire ant, Solenopsis invicta Buren

The genetic variability and population structure of monogynous (M) and polygynous (P) forms of Solenopsis invicta Buren, the red imported fire ant (IFA), were examined using starch gel electrophoresis of isozymes. The study focused on collections from twelve Texas counties; samples from Louisiana, Mississippi, Alabama, and Georgia were examined for comparison. Six of the thirty-one Texas collecting sites had sympatric monogynous and polygynous IFA populations.; Three previously undetected polymorphic enzymes were resolved: hexokinase (HK), phosphoglucomutase (PGM), and aminopeptidase (AP). Both HK and AP have an electromorph which is common in the P form but is not present in M individuals. The genetic basis of the HK and PGM variants was confirmed; that of the AP variants was not.; Five polymorphic loci and 21 presumptive monomorphic loci were used to study IFA population structure. The polymorphic loci Hk-1, Hk-2 and Pgm-1 were all described for the first time from S. invicta in this study. Esterase-4 and {dollar}alpha{dollar}-glycerophosphate dehydrogenase-1, two variable loci that had been described previously, were also used. Two alleles were found at each of the polymorphis loci, and allele frequencies were near the theoretical maximum of 0.5. Hexokinase-2 was polymorphic in the P form, and monomorphic in the M form. Monogynous and polygynous samples from other states had electrophoretic patterns that were indistinguishable from their Texas counterparts. The proportion of polymorphic loci was 0.19 in S. invicta and in the P form, and 0.16 in the M form. Average heterozygosity was 0.078 in S. invicta, 0.086 in the P form and 0.069 in the M form.; There was no evidence of inbreeding in either monogynous or polygnous IFA. Relatedness in the M form was estimated to be 0.678, close to the theoretical maximum of 0.75 for singly mated, single queen colonies. The P relatedness estimate was 0.211, suggesting that only a few queens in a P colony may produce the colony's reproductive progeny. There was no indication of differentiation within either IFA form. There was, however, moderate differentiation and therefore restricted gene flow between M and P IFA, as indicated by the 0.059 between forms {dollar}Fsb{lcub}rm ST{rcub}{dollar} estimate.; A survey of isozyme variability in three other fire ant species, Solenopsis geminata, S. richteri, and S. xyloni, indicated that the unique polygynous HK and AP isozymes did not appear as a result of hybridization of S. invicta with one of these species.