Polygyne Form Research Articles

Transient effects of corridors on polygyne fire ants over a decade

Abstract Although corridors are frequently regarded as a way to mitigate the negative effects of habitat fragmentation, concerns persist that corridors may facilitate the spread of invasive species to the detriment of native species. The invasive fire ant, Solenopsis invicta, has two social forms. The polygyne form has limited dispersal abilities relative to the monogyne form. Our previous work in a large‐scale corridor experiment showed that in landscapes dominated by the polygyne form, fire ant density was higher and native ant species richness was lower in habitat patches connected by corridors than in unconnected patches. We expected that these observed corridor effects would be transient, that is, that fire ant density and native ant species richness differences between connected and unconnected patches would diminish over time as fire ants eventually fully established within patches. We tested this prediction by resampling the three landscapes dominated by polygyne fire ants 6 to 11 years after our original study. Differences in fire ant density between connected and unconnected habitat patches in these landscapes decreased, as expected. Differences in native ant species richness were variable but lowest in the last 2 years of sampling. These findings support our prediction of transient corridor effects on this invasive ant and stress the importance of temporal dynamics in assessing population and community impacts of habitat connectivity.

Effects of social organization and elevation on spatial genetic structure in a montane ant

Studying patterns of population structure across the landscape sheds light on dispersal and demographic processes, which helps to inform conservation decisions. Here, we study how social organization and landscape factors affect spatial patterns of genetic differentiation in an ant species living in mountainous regions. Using genome‐wide SNP markers, we assess population structure in the Alpine silver ant, Formica selysi. This species has two social forms controlled by a supergene. The monogyne form has one queen per colony, while the polygyne form has multiple queens per colony. The two social forms co‐occur in the same populations. For both social forms, we found a strong pattern of isolation‐by‐distance across the Alps. Within regions, genetic differentiation between populations was weaker for the monogyne form than for the polygyne form. We suggest that this pattern is due to higher dispersal and effective population sizes in the monogyne form. In addition, we found stronger isolation‐by‐distance and lower genetic diversity in high elevation populations, compared to lowland populations, suggesting that gene flow between F. selysi populations in the Alps occurs mostly through riparian corridors along lowland valleys. Overall, this survey highlights the need to consider intraspecific polymorphisms when assessing population connectivity and calls for special attention to the conservation of lowland habitats in mountain regions.

Characterization of Queen Supergene Pheromone in the Red Imported Fire Ant Using Worker Discrimination Assays

Ants use chemical signals to communicate for various purposes related to colony function. Social organization in the red imported fire ant, Solenopsis invicta, is determined by the Sb supergene, with colonies of the monogyne (single-queen) form lacking the element and colonies of the polygyne (multiple-queen) form possessing it. Polygyne workers accept new reproductive queens in their nest, but only those carrying Sb; young winged queens lacking this genetic element are executed as they mature sexually in their natal nest or as they attempt to enter a foreign nest to initiate reproduction after mating and shedding their wings. It has been suggested that queen supergene genotype status is signaled to workers by unsaturated cuticular hydrocarbons, while queen reproductive status is signaled by piperidines (venom alkaloids). We used high-throughput behavioral assays to study worker acceptance of paper dummies dosed with fractions of extracts of polygyne queens, or blends of synthetic counterparts of queen cuticular compounds. We show that the queen supergene pheromone comprises a blend of monoene and diene unsaturated hydrocarbons. Our assays also reveal that unsaturated hydrocarbons elicit discrimination by polygyne workers only when associated with additional compounds that signal queen fertility. This synergistic effect was obtained with a polar fraction of queen extracts, but not by the piperidine alkaloids, suggesting that the chemical(s) indicating queen reproductive status are compounds more polar than cuticular hydrocarbons but are not the piperidine alkaloids. Our results advance understanding of the role of chemical signaling that is central to the regulation of social organization in an important invasive pest and model ant species.

The Insecticidal Activities of Fire Ant (Hymenoptera: Formicidae) Venoms Against <I>Plutella xylostella</I> (Lepidoptera: Plutellidae) Larvae

Although fire ants frequently have negative impacts on agricultural systems and public health, they have additional beneficial insecticidal effects. To evaluate the potential effect of fire ant venoms on agricultural pests, the compositions of the venoms and their insecticidal activities against Plutella xylostella (L.) larvae were evaluated under laboratory conditions. The alkaloids found in Solenopsis geminata (F.) venom are primarily saturated C11, which occur in both cis and trans forms, whereas the venom of S. invicta Buren contains six principal alkaloids (from trans C1, to C17). Moreover, the proportions of unsaturated alkaloids in the venom of polygynous S. invicta were significantly higher than the corresponding proportions in the monogynous S. invicta, as shown by our previous studies. Fire ant venoms were topically applied to the dorsal thoracic region of fourth-instar larvae of P. xylostella. The results of the experiment showed that the larval symptoms induced by fire ant venom include contractile, flaccid paralysis, black coloration and death. P. xylostella larvae were most susceptible to S. geminata venom. The order of the susceptibilities of the larvae to the venoms was as follows: S. geminata > S. invicta (monogyne form) > S. invicta (polygyne form), as measured by the corresponding LT50 values at 24 h.

Behavioral Discrimination between Monogyne and Polygyne Red Fire Ants (Hymenoptera: Formicidae) in their Native Range

Abstract Nestmate recognition among social insects is presumed to restrict non-nestmates from exploiting nest resources. Here, we developed aggression bioassays to assess the discrimination behaviors of both polygynous and monogynous forms of the red fire ant, Solenopsis invicta Buren, during symmetrical interactions in neutral arenas. Workers from polygyne colonies exhibited risk avoidance behaviors; that is, defensive postures or the avoidance of direct contact during interactions. Workers from monogyne colonies always exhibited aggressive behaviors in the form of physical or chemical attacks. In interactions between both, monogyne workers usually started the aggression by surrounding and biting the polygyne ants. Polygyne S.invicta workers also distinguished nestmates from foreigners, but their response was not as aggressive as that of monogynes. The proposed ethogram that we constructed identified monogyne and polygyne forms of S.invicta colonies in concordance with current measures, including number of queens, and expression of the Gp-9 gene.

Venom Alkaloid and Cuticular Hydrocarbon Profiles Are Associated with Social Organization, Queen Fertility Status, and Queen Genotype in the Fire Ant Solenopsis invicta

Queens in social insect colonies advertise their presence in the colony to: a) attract workers' attention and care; b) gain acceptance by workers as replacement or supplemental reproductives; c) prevent reproductive development in nestmates. We analyzed the chemical content of whole body surface extracts of adult queens of different developmental and reproductive stages, and of adult workers from monogyne (single colony queen) and polygyne (multiple colony queens) forms of the fire ant Solenopsis invicta. We found that the composition of the most abundant components, venom alkaloids, differed between queens and workers, as well as between reproductive and non-reproductive queens. Additionally, workers of the two forms could be distinguished by alkaloid composition. Finally, sexually mature, non-reproductive queens from polygyne colonies differed in their proportions of cis-piperidine alkaloids, depending on their Gp-9 genotype, although the difference disappeared once they became functional reproductives. Among the unsaturated cuticular hydrocarbons characteristic of queens, there were differences in amounts of alkenes/alkadienes between non-reproductive polygyne queens of different Gp-9 genotypes, between non-reproductive and reproductive queens, and between polygyne and monogyne reproductive queens, with the amounts increasing at a relatively higher rate through reproductive ontogeny in queens bearing the Gp-9 b allele. Given that the genotype-specific piperidine differences reflect differences in rates of reproductive maturation between queens, we speculate that these abundant and unique compounds have been co-opted to serve in fertility signaling, while the cuticular hydrocarbons now play a complementary role in regulation of social organization by signaling queen Gp-9 genotype.

Current Status of a Model System: The Gene Gp-9 and Its Association with Social Organization in Fire Ants

The Gp-9 gene in fire ants represents an important model system for studying the evolution of social organization in insects as well as a rich source of information relevant to other major evolutionary topics. An important feature of this system is that polymorphism in social organization is completely associated with allelic variation at Gp-9, such that single-queen colonies (monogyne form) include only inhabitants bearing B-like alleles while multiple-queen colonies (polygyne form) additionally include inhabitants bearing b-like alleles. A recent study of this system by Leal and Ishida (2008) made two major claims, the validity and significance of which we examine here. After reviewing existing literature, analyzing the methods and results of Leal and Ishida (2008), and generating new data from one of their study sites, we conclude that their claim that polygyny can occur in Solenopsis invicta in the U.S.A. in the absence of expression of the b-like allele Gp-9b is unfounded. Moreover, we argue that available information on insect OBPs (the family of proteins to which GP-9 belongs), on the evolutionary/population genetics of Gp-9, and on pheromonal/behavioral control of fire ant colony queen number fails to support their view that GP-9 plays no role in the chemosensory-mediated communication that underpins regulation of social organization. Our analyses lead us to conclude that there are no new reasons to question the existing consensus view of the Gp-9 system outlined in Gotzek and Ross (2007).

Venom alkaloids of monogyne and polygyne forms of the red imported fire ant, Solenopsis invicta, in Taiwan

Both monogyne and polygyne colonies of Solenopsis invicta now occupy Taiwan. Although venom alkaloids of these ants have been described and synthesized, we here report on a quantitative analysis of the two social forms for the first time. The alkaloids were studied by gas chromatography coupled to mass spectrometry (GC-MS), and six major venom alkaloids were detectable in both types of workers. Both C13:C13:1 and C15:C15:1 ratios in alkaloid venom of monogyne workers were statistically higher than that of polygyne workers, but the sum of proportions of unsaturated alkaloids of polygyne workers was significantly higher than that of monogyne workers, regardless of growth temperature, sampling seasons or geographic location. Results of this study demonstrate that the difference in the proportions of unsaturated alkaloids and the ratios of C13:C13:1 and C15:C15:1 alkaloids might be a good indicator for differentiating monogyne and polygyne forms of S. invicta.

Molecular variation at a candidate gene implicated in the regulation of fire ant social behavior.

The fire ant Solenopsis invicta and its close relatives display an important social polymorphism involving differences in colony queen number. Colonies are headed by either a single reproductive queen (monogyne form) or multiple queens (polygyne form). This variation in social organization is associated with variation at the gene Gp-9, with monogyne colonies harboring only B-like allelic variants and polygyne colonies always containing b-like variants as well. We describe naturally occurring variation at Gp-9 in fire ants based on 185 full-length sequences, 136 of which were obtained from S. invicta collected over much of its native range. While there is little overall differentiation between most of the numerous alleles observed, a surprising amount is found in the coding regions of the gene, with such substitutions usually causing amino acid replacements. This elevated coding-region variation may result from a lack of negative selection acting to constrain amino acid replacements over much of the protein, different mutation rates or biases in coding and non-coding sequences, negative selection acting with greater strength on non-coding than coding regions, and/or positive selection acting on the protein. Formal selection analyses provide evidence that the latter force played an important role in the basal b-like lineages coincident with the emergence of polygyny. While our data set reveals considerable paraphyly and polyphyly of S. invicta sequences with respect to those of other fire ant species, the b-like alleles of the socially polymorphic species are monophyletic. An expanded analysis of colonies containing alleles of this clade confirmed the invariant link between their presence and expression of polygyny. Finally, our discovery of several unique alleles bearing various combinations of b-like and B-like codons allows us to conclude that no single b-like residue is completely predictive of polygyne behavior and, thus, potentially causally involved in its expression. Rather, all three typical b-like residues appear to be necessary.

Modelling the potential spread of Solenopsis invicta Buren (Hymenoptera: Formicidae) (red imported fire ant) in Australia

Quantifying the potential spread and density of an invading organism enables decision-makers to determine the most appropriate response to incursions. We present two linked models that estimate the spread of Solenopsis invicta Buren (red imported fire ant) in Australia based on limited data gathered after its discovery in Brisbane in 2001. A stochastic cellular automaton determines spread within a location (100 km by 100 km) and this is coupled with a model that simulates human-mediated movement of S. invicta to new locations. In the absence of any control measures, the models predict that S. invicta could cover 763 000–4 066 000 km2 by the year 2035 and be found at 200 separate locations around Australia by 2017–2027, depending on the rate of spread. These estimated rates of expansion (assuming no control efforts were in place) are higher than those experienced in the USA in the 1940s during the early invasion phases in that country. Active control efforts and quarantine controls in the USA (including a concerted eradication attempt in the 1960s) may have slowed spread. Further, milder winters, the presence of the polygynous social form, increased trade and human mobility in Australia in 2000s compared with the USA in 1940s could contribute to faster range expansion.

Molecular Evolutionary Analyses of the Odorant-Binding Protein Gene Gp-9 in Fire Ants and Other Solenopsis Species

The fire ant Solenopsis invicta exists in two social forms, one with colonies headed by a single reproductive queen (monogyne form) and the other with colonies containing multiple queens (polygyne form). This variation in social organization is associated with variation at the gene Gp-9, with monogyne colonies harboring only the B allelic variant and polygyne colonies containing b-like variants as well. We generated new Gp-9 sequences from 15 Solenopsis species and combined these with previously published sequences to conduct a comprehensive, phylogenetically based study of the molecular evolution of this important gene. The exon/intron structure and the respective lengths of the five exons of Gp-9 are identical across all species examined, and we detected no evidence for intragenic recombination. These data conform to a previous suggestion that Gp-9 lies in a genomic region with low recombination, and they indicate that evolution of the coding region in Solenopsis has involved point substitutions only. Our results confirm a link between the presence of b-like alleles and the expression of polygyny in all South American fire ant species known to possess colonies of both social forms. Moreover, phylogenetic analyses show that b-like alleles comprise a derived clade of Gp-9 sequences within the socially polymorphic species, lending further support to the hypothesis that monogyny preceded polygyny in this group of fire ants. Site-specific maximum likelihood tests identified several amino acids that have experienced positive selection, two of which are adjacent to the inferred binding-pocket residues in the GP-9 protein. Four other binding-pocket residues are variable among fire ant species, although selection is not implicated in this variation. Branch-specific tests revealed strong positive selection on the stem lineage of the b-like allele clade, as expected if selection drove the amino acid replacements crucial to the expression of polygyne social organization. Such selection may have operated via the ligand-binding properties of GP-9, as one of the two amino acids uniquely shared by all b-like alleles is predicted to be a binding-pocket residue.

Prevalence of <I>Thelohania solenopsae</I> (Microsporidia: Thelohaniidae) Infection in Monogyne and Polygyne Red Imported Fire Ants (Hymenoptera: Formicidae)

Abstract We determined the prevalence of natural field infections of the fire ant pathogen Thelohania solenopsae Knell, Allen, and Hazard in the monogyne and polygyne social forms of the red imported fire ant, Solenopsis invicta Buren, in three pastures in Florida. Social form was determined by examining the genotype of ants at the Gp-9 locus. The monogyne form contains a single fertilized queen per colony, and colony members have a genotype of Gp-9BB. In contrast, the polygyne form contains many fertilized queens per colony, and all three genotypes of Gp-9BB, Gp-9Bb, or Gp-9bb can be present within a colony. Among the study sites, ratios of monogyne:polygyne colonies ranged from 3:55–28:22, and infections rates were 42–78% when both social forms were included in the samples from each site. However, T. solenopsae infections were restricted to colonies of the polygynous social form. While T. solenopsae was only detected in polygynous colonies in the field, T. solenopsae infections were found in ants with t...

Distribution and Prevalence of <I>Wolbachia</I> Infections in Native Populations of the Fire Ant <I>Solenopsis invicta</I> (Hymenoptera: Formicidae)

Wolbachia are endosymbiotic bacteria that commonly infect arthropods. These bacteria induce a number of phenotypes in their hosts, including cytoplasmic incompatibility, thelytokous parthenogenesis, feminization, and male killing. We surveyed native South American populations of the fire ant Solenopsis invicta Buren for Wolbachia infections by using a diagnostic polymerase chain reaction assay. In addition, we determined the fidelity of vertical transmission of the bacteria from mother to offspring in this species by assaying daughters in 24 simple-family (monogyne) colonies. Infections were common in many parts of the extensive native range of S. invicta. However, the proportion of individuals infected varied greatly among samples, ranging from zero in several populations from the northerly parts of the range to >90% in more southerly populations. Possible explanations for this variation in the prevalence of Wolbachia infections are discussed. A survey of the two social forms of S. invicta from four geographic areas showed that the prevalence of Wolbachia infections consistently was higher in the monogyne form (single queen per colony) than the sympatric polygyne form (multiple queens per colony). One likely explanation for this trend is that the selective regimes acting on Wolbachia in the two forms differ because of the dissimilar reproductive strategies used by each form. Finally, overall transmission efficiency was found to be very high (>99%), making it unlikely that imperfect transmission prevents the spread of the microbe to near fixation in native populations.

Sympatry of Polygyne and Monogyne Colonies of the Fire Ant <I>Solenopsis invicta</I> (Hymenoptera: Formicidae)

The polygyne form of the red imported fire ant, Solenopsis invicta Buren, is thought to occur primarily in discrete populations embedded within areas composed of monogyne colonies. This distribution implies that polygyne colonies compete with monogyne colonies and subsequently create a population homogenous in social form. Because polygyne colonies produce mostly sterile males, opportunities for insemination of female alates might be minimal at the centers of large polygyne areas. To test for homogeneity in social form, a large polygyne population described in northcentral Florida was examined for the presence of single queen colonies. Forty-six to 51 colonies were sampled from each of six sites located along an east-west transect through the middle of the polygyne area in Marion County and a site at the northernmost limit of the polygyne area, Gainesville, FL. Several hundred workers from each nest were collected along with nest material. The social form of each sample was determined by an aggression test of workers to the introduction of non-nestmates and by the dissection of males for sterility. Both social forms of S. invicta were present at all collection sites and 103 of 333 (30.93%) colonies sampled were determined to be monogyne colonies. Among the collection sites, the percentage of colonies that were monogyne ranged from 3.9 to 57.4%. The polygyne region in northcentral Florida is more accurately described as an area where relatively high frequencies of polygyne colonies are interdispersed with single queen colonies.

A comparison of the colony-founding potential of queens from single- and multiple-queen colonies of the fire antSolenopsis invicta

Newly mated queens from the polygyne (multiple-queen) form of S. invicta show a weight polymorphism that correlates with their genotype at the protein locus Gp-9. Although this variation in weight might be expected to translate into variation in the ability of queens to initiate new colonies using stored energy reserves, a systematic examination of the colony-founding ability of newly mated polygyne-derived queens of different weights has never been reported. Here I compare the ability of monogyne-derived queens (Gp-9BB M), heavy polygyne-derived queens (Gp-9BB P), and light polygyne-derived queens (Gp-9Bb P) to initiate their own colonies using only stored energy reserves. Most measurements of the ants' abilities yielded the following scale of competency:Gp-9BB M>Gp-9BB P>Gp-9Bb P. Surprisingly, most mated polygyne-derived queens of even the lighter genotype were capable of rearing considerable numbers of workers in isolation. This ability may be enhanced substantially in the field if such queens cooperate in initiating new nests (pleometrosis). These results are concordant with the growing body of work that implicates a simply inherited genetic polymorphism for the control of a complex social trait in this ant, and they indicate that the modes of reproduction in polygyne fire ants may show considerable diversity.Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved .

Queen Attractiveness to Workers in the Polygynous Form of the Ant Solenopsis invicta (Hymenoptera: Formicidae)

We found that queens in laboratory colonies of the polygynous form of the imported fire ant, Solenopsis invicta Buren, are aggregated. No fighting or competitive behavioral interactions between queens were observed. We then examined the response of workers to individual queens in an effort to determine if queens within the same colony were equal in worker attractiveness. When queens were separated and workers were allowed to choose among them, 1 queen generally attracted >70% of the workers and brood. However, the difference in worker numbers attracted to remaining queens in the colony was not significant. The difference in queen's attractiveness to workers might be the result of a quantitative variation of queen pheromone. Increasing the number of queens and size of the colonies still resulted in most workers consistently responding to one of the queens. When the queen that attracted >70% of the workers was removed, >70% of the workers responded to another queen. When the removed queen was returned to the colony, she was again chosen by >70% of the workers.

Parasitoid–host relationships when host size varies: the case of Pseudacteon flies and Solenopsis fire ants

Abstract.1. Phorid flies in the genus Pseudacteon are parasitoids of ants. Variation in host size preferences of four South American and two North American Pseudacteon species on monogyne and polygyne forms of their host Solenopsis species [S. invicta Buren and S. geminata (F.), respectively] was documented.2. Monogyne Solenopsis workers were, on average, significantly larger than polygyne workers, and the average size monogyne worker attacked was significantly larger than the average size polygyne worker attacked in four of the six Pseudacteon species.3. Three South American Pseudacteon species attacked larger than average size workers, whereas one attacked smaller than average size workers, in both monogyne and polygyne forms. Both North American Pseudacteon species attacked larger than average size polygyne workers and smaller than average size monogyne workers.4. Three Pseudacteon species were reared from eggs to adults in infected ants in the laboratory. The size of the emergent phorid fly was related positively to the size of the host worker ant, with females emerging from larger hosts. Similar patterns were documented for both monogyne and polygyne forms.5. The mean size of worker host from which phorids emerged did not differ significantly between the monogyne and polygyne forms in the subsample of phorids reared to adults.6. The observed patterns elucidate factors that may cause variation in Pseudacteon sex ratios, and have implications for biological control efforts of pest Solenopsis species.

Hierarchical analysis of genetic structure in native fire ant populations: results from three classes of molecular markers.

We describe genetic structure at various scales in native populations of the fire ant Solenopsis invicta using two classes of nuclear markers, allozymes and microsatellites, and markers of the mitochondrial genome. Strong structure was found at the nest level in both the monogyne (single queen) and polygyne (multiple queen) social forms using allozymes. Weak but significant microgeographic structure was detected above the nest level in polygyne populations but not in monogyne populations using both classes of nuclear markers. Pronounced mitochondrial DNA (mtDNA) differentiation was evident also at this level in the polygyne form only. These microgeographic patterns are expected because polygyny in ants is associated with restricted local gene flow due mainly to limited vagility of queens. Weak but significant nuclear differentiation was detected between sympatric social forms, and strong mtDNA differentiation also was found at this level. Thus, queens of each form seem unable to establish themselves in nests of the alternate type, and some degree of assortative mating by form may exist as well. Strong differentiation was found between the two study regions using all three sets of markers. Phylogeographic analyses of the mtDNA suggest that recent limitations on gene flow rather than longstanding barriers to dispersal are responsible for this large-scale structure.

Nuclear and mitochondrial genetic structure in two social forms of the fire ant Solenopsis invicta: insights into transitions to an alternate social organization

We describe genetic structure at microgeographical scales in two social forms of the introduced fire ant Solenopsis invicta using nuclear and mitochondrial markers. Fire ant populations in northern Georgia, U.S.A. are structured in fundamentally different ways at the two genomes. Virtually all nuclear genetic variation resides within sampling sites; no differentiation was apparent between the social forms and only minimal differentiation was found among sites in only one social form (the polygyne form, which has nests with numerous queens). In contrast, substantial mtDNA variation exists at all levels; the forms are significantly differentiated and strong divergence was detected among sites in the polygyne form. No mtDNA differentiation was found among sites in the monogyne form (which has nests with a single queen). These results are predicted on the basis of the social biology of the ants. Monogyne queens and males of both forms presumably disperse widely, and polygyne queens commonly mate with monogyne males, thus explaining the uniformity of nuclear allele frequencies across sampling sites and between the forms. Polygyne queens display habits consistent with limited dispersal, and queens of both forms seem unable to establish themselves as reproductives in nests of the alternate form, thus explaining the strong microgeographical structure within the polygyne form and differentiation between the forms found using mtDNA markers. The mtDNA haplotypes are distributed among sites in a manner that suggests a mechanism for the spread of polygyny in introduced fire ants. Monogyne queens apparently mate occasionally with polygyne males and acquire a Mendelian nuclear factor allowing the expression of polygyny in their sexual daughters.

Nuclear and mitochondrial genetic structure in two social forms of the fire ant Solenopsis invicta: insights into transitions to an alternate social organization

We describe genetic structure at microgeographical scales in two social forms of the introduced fire ant Solenopsis invicta using nuclear and mitochondrial markers. Fire ant populations in northern Georgia, U.S.A. are structured in fundamentally different ways at the two genomes. Virtually all nuclear genetic variation resides within sampling sites; no differentiation was apparent between the social forms and only minimal differentiation was found among sites in only one social form (the polygyne form, which has nests with numerous queens). In contrast, substantial mtDNA variation exists at all levels; the forms are significantly differentiated and strong divergence was detected among sites in the polygyne form. No mtDNA differentiation was found among sites in the monogyne form (which has nests with a single queen). These results are predicted on the basis of the social biology of the ants. Monogyne queens and males of both forms presumably disperse widely, and polygyne queens commonly mate with monogyne males, thus explaining the uniformity of nuclear allele frequencies across sampling sites and between the forms. Polygyne queens display habits consistent with limited dispersal, and queens of both forms seem unable to establish themselves as reproductives in nests of the alternate form, thus explaining the strong microgeographical structure within the polygyne form and differentiation between the forms found using mtDNA markers. The mtDNA haplotypes are distributed among sites in a manner that suggests a mechanism for the spread of polygyny in introduced fire ants. Monogyne queens apparently mate occasionally with polygyne males and acquire a Mendelian nuclear factor allowing the expression of polygyny in their sexual daughters.