A formal assessment of gene flow and selection in the fire ant Solenopsis invicta

Abstract.— Recent studies of the introduced fire ant Solenopsis invicta suggest that introduced polygyne (with multiple queens per nest) populations are strongly influenced by male-mediated gene flow from neighboring monogyne (single queen per nest) populations and selection acting on a single locus, general protein-9 (Gp-9) . This investigation formally tests this hypothesis and determines if these processes can account for the genotypic structure of polygyne S. invicta . To increase the statistical power of this test, we considered the genotypes of polygyne queens and workers at both Gp-9 and the closely linked, selectively neutral locus Pgm-3 . We then constructed and analyzed a novel mathematical model to delimit the effects of monogyne male gene flow and selection on the joint genotypes at the Pgm-3/Gp-9 superlocus. Using this framework, a hierarchical maximum-likelihood method was developed to estimate the best-fitting gene flow and selection parameters based on the fit of our model to data from both the current study and an earlier one of the same population. In each case, selection on polygyne queens and workers alone, with no monogyne male gene flow, provides the most parsimonious explanation for the observed genotype frequencies. The apparent discrepancy between this result and the empirical evidence for monogyne male gene flow indicates that undocumented factors, such as other forms of selection in polygyne males or workers, are operating in introduced polygyne S. invicta .     

Enzyme polymorphisms in the fire ant,Solenopsis invicta (hymenoptera: formicidae)

A survey of allozyme variability in the fire ant, Solenopsis invicta Buren, was conducted using samples collected throughout the species' range in North America. A substantial amount of variability was found, although heterozygosity estimates were considerably less than estimates for diploid non-hymenopterous species. The presence of this variability raises the possibility of multiple introductions of this ant to North America. Although based on a limited sample, our data indicate no differences in variability between individuals from monogynous and polygynous populations or between individuals from the Mobile, Alabama area (presumed point of introduction) and outside the area. Individuals from polygynous colonies shared all detected alleles with individuals from monogynous colonies indicating the lack of long-term reproductive isolation between the two types of populations.     

Microsatellite variation in natural Drosophila melanogaster populations from New South Wales (Australia) and Tasmania

Microsatellite variation was studied at 48 microsatellite loci in 10 Drosophila melanogaster populations to investigate the population structure on the Australian east coast. Low, but statistically significant population differentiation was observed among most populations. The populations on the Australian mainland did not show evidence for isolation by distance. We conclude that the population structure of D. melanogaster on the Australian mainland is probably the result of a shared history (recent colonization). The observed differences between local D. melanogaster populations probably reflect variation in effective population sizes rather than patterns of gene flow. Two populations from Tasmania were more differentiated from the Australian mainland than a population from Israel, raising the question whether they are derived from the Australian mainland or colonized from a different source population.     

Population genetic structure and gene flow patterns between populations of the Antarctic icefish Chionodraco rastrospinosus

Aim A lack of genetic structure is predicted for Antarctic fish due to the duration of pelagic larval stages and the strength of the currents in the Southern Ocean, particularly the Antarctic Circumpolar Current. In this study we explored the population structure of the ocellated icefish, Chionodraco rastrospinosus, by means of analysing a total of 394 individuals collected at four geographical areas off the Antarctic Peninsula in the period 1996–2006. Location Elephant Island, southern South Shetlands, Joinville Island and South Orkneys in the Southern Ocean. Methods The spatio‐temporal genetic structure of Chionodraco rastrospinosus was explored using seven microsatellite loci. Existence and direction of gene flow across sampling locations were investigated using the isolation‐by‐migration procedure. Results Microsatellite data showed a lack of genetic structuring in the area studied, with no differences found at both the geographical or temporal level, and an eastward unidirectional gene flow among sites. This suggested a lack of genetic barriers for this species, attributable to larval dispersal following the Antarctic Circumpolar Current, which fits well with the predicted pattern for Antarctic fish. Re‐examination of genetic data of the closely related icefish Chaenocephalus aceratus, with similar larval duration but displaying genetically structured populations, indicated a weak but significant bidirectional gene flow. Main conclusions Our results point to a relationship that is more complex than expected between potential for dispersal and realized gene flow in the marine environment. In addition to ocean circulation and larval dispersal, other major life‐history traits might be driving connectivity, particularly larval retention.     

Genetic variation and structure in Scandinavian red deer (Cervus elaphus): influence of ancestry,past hunting,and restoration management

In the 19^th century, the red deer (Cervus elaphus) population in Sweden experienced a rapid decline in numbers and distribution. A small population was, however, remnant in the southernmost province (Skåne) of the country, presumably corresponding to the nominate form of red deer (Cervus elaphus elaphus Linnaeus, 1758). After management, reintroductions, and supplementary release during the 20^th century the Swedish C. elaphus population recovered. The recovery was partially uncontrolled, and included introductions of C. elaphus of continental origin. In northern central Sweden (Jämtland) the current C. elaphus population may stem from natural colonization from Norway and/or from specimens of Swedish origin that have escaped from enclosures. To evaluate the status of the current, partially separated populations, we investigated variation at microsatellite markers in 157 C. elaphus specimens from ten locations in Sweden and Norway. Analyses suggest that the highest‐likelihood phylogenetic structure among the individuals sampled is described four distinct genetic clusters: (1) animals from the province of Västergötland in south‐western Sweden; (2) deer from the southernmost province of Skåne; (3) deer from the provinces Jämtland, Blekinge, and Västmanland; and (4) Norwegian deer. Cervus elaphus from a captive herd at the Skåne Zoo cluster with deer from Skåne or deer from Västergötland, depending on the method of analysis. A number of populations in Sweden may genetically match the nominate form of red deer (C. e. elaphus). The recently established C. elaphus population in Jämtland seems to stem mainly from escapees from enclosures, with a mixed ancestry from the wild remnant population in Skåne and continental deer, whereas the influx from Norway is minor, if any. Our results show the need for a detailed assessment of genetic differentiation, and emphasize the value of local management plans when planning and managing introductions. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 109 , 43–53.     

Genetic structure and population differentiation of the Mediterranean pioneer spiny broom Calicotome villosa across the Strait of Gibraltar

The region around the Strait of Gibraltar is considered to be one of the most relevant 'hot spots' of biodiversity in the Mediterranean Basin due to its historical, biogeographical, and ecological features. Prominent among these is its role as a land bridge for the migration and differentiation of species during the Pleistocene, as a consequence of the lowering of sea level and climate changes associated with the Ice Ages. In the present study, we report a multilevel hierarchical investigation of the genetic diversity of Calicotome villosa , a common pioneer legume shrub, at the regional scale. The results of genetic analysis of progeny arrays are consistent with a predominantly outcrossing mating system in all the populations analysed. Geographically, a pattern of population isolation by distance was found, but the Strait accounted for only approximately 2% of the among-population genetic differentiation. Consequently, extensive historical gene flow appears to be the rule for this species in this area. According to the natural history traits of C. villosa (pollination, dispersal, and colonization ability), we hypothesize that gene flow must be strongly influenced by seed dispersal because pollen flow is very limited. Based on the history of trade and land use, cattle and human movements across the Strait must have strongly favoured seed dispersal. We review and discuss these results and compare them with those of other reported studies of genetic and phylogenetic differentiation across the Strait of Gibraltar. It is stressed that colonization ability, which depends upon seed dispersal and life form, can be a more critical factor in gene flow than pollination.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 39–51.     

Estimation of the number of founders of an invasive pest insect population: the fire ant Solenopsis invicta in the USA

Determination of the number of founders responsible for the establishment of invasive populations is important for developing biologically based management practices, predicting the invasive potential of species, and making inferences about ecological and evolutionary processes. The fire ant Solenopsis invicta is a major invasive pest insect first introduced into the USA from its native South American range in the mid-1930s. We use data from diverse genetic markers surveyed in the source population and the USA to estimate the number of founders of this introduced population. Data from different classes of nuclear markers (microsatellites, allozymes, sex-determination locus) and mitochondrial DNA are largely congruent in suggesting that 9-20 unrelated mated queens comprised the initial founder group to colonize the USA at Mobile, Alabama. Estimates of founder group size based on expanded samples from throughout the southern USA were marginally higher than this, consistent with the hypothesis of one or more secondary introductions of the ant into the USA. The rapid spread and massive population build-up of introduced S. invicta occurred despite the loss of substantial genetic variation associated with the relatively small invasive propagule size, a pattern especially surprising in light of the substantial genetic load imposed by the loss of variation at the sex-determination locus.     

Sex investment ratios in monogyne and polygyne populations of the fire ant Solenopsis invicta

Both monogyne (single queen per colony) and polygyne (multiple queens per colony) populations of the fire ant Solenopsis invicta are good subjects for tests of kin selection theory because their genetic and reproductive attributes are well-characterized, permitting quantitative predictions about the degree to which sex investment ratios should be female-biased if workers and not queens control reproductive allocation. In the study populations, an investment ratio of 3 females: 1 male is predicted (a proportional investment in females of 0.75) in the monogyne form, whereas a proportional investment in females between 0.637 and 0.740 is expected in the polygyne form. To test these predictions, colonies from a single population of each social form were collected and censused during three different seasons. Consistent with their alternative modes of colony founding, monogyne colonies invested more in reproduction (sexual production) and less in growth/maintenance (worker production) than did the polygyne colonies. Overall, the sex investment ratios were female-biased in both forms, although there was considerable seasonal variation. After adjusting for sex-specific energetic costs, the proportional investment in females was 0.607 in the monogyne population, a value in between those expected under complete control by either the queen or the workers. However, when combined with data from four other previously studied monogyne populations in the U.S.A., the mean investment ratio did not differ significantly from the value predicted if workers have exclusive control. In the polygyne population, the proportional investment in females of 0.616 was consistent with the level of female bias expected under partial to complete worker control, although the potential influence of two confounding factors — possible contact with monogyne colonies and the preponderance of sterile diploid males — weakens this conclusion somewhat. Taken as a whole, the sex investment ratios of monogyne and polygyne populations of S. invicta are consistent with at least partial worker control. Of several ultimate and proximate explanations that have been proposed to explain inter-colonial variation in the sex investment ratio, only the effect of the primary sex ratio (female-determined eggs: male-determined eggs) laid by the queen appears to account for the observed variation among monogyne colonies. In the polygyne population, there is limited support for the hypothesis that greater resource abundance favors investment in females.     

Genetic variation in original and colonizing Drosophila buzzatii populations analysed by microsatellite loci isolated with a new PCR screening method

A new polymerase chain reaction-based screening method for microsatellites is presented. Using this method, we isolated 12 microsatellite loci from Drosophila buzzatii, two of which were X-linked. We applied the other 10 microsatellite loci to the analysis of genetic variation in five natural populations of D. buzzatii. Two populations were from the species' original distribution in Argentina, whereas the other three were from Europe (two) and Australia that were colonized 200 and 65 years ago, respectively. Allelic variation was much larger in the original populations than in the colonizing ones and there was a tendency to decreased heterozygosity in the colonizing populations. We used three different statistical procedures for detecting population bottlenecks. All procedures suggested that the low variability in the populations in the Old World was not the result of the recent population decline, but was due to a founder effect followed by a population expansion. In fact, one procedure which detects population expansions and declines based on the genealogical history of microsatellite data suggested that an expansion had taken place in all the colonized populations.     

Mitochondrial DNA variation among populations of Oedura reticulata (Gekkonidae) in remnant vegetation: implications for metapopulation structure and population decline

Restriction fragment length polymorphism (RFLP) analysis of mitochondrial DNA was used to examine the regional population structure of a species of gecko (Oedura reticulata) in vegetation remnants within the Western Australian wheatbelt. The species exhibited considerable polymorphism within and between populations with 22 haplotypes recognized among 12 populations. Phylogenetic analysis of haplotypes and clustering of nucleotide divergence among populations demonstrated little regional structure within the species with several haplotypes present in all three regions surveyed. This contrasted markedly with variation in haplotype frequency among populations which showed a high degree of independence between populations indicating that current levels of maternal gene flow are low and that the populations are too small to prevent genetic drift. This conclusion is supported by generally lower numbers of haplotypes in remnant populations than in nearby nature reserves. These findings, combined with demographic data from a previous study, suggest that post-fragmentation populations of O. reticulata are unable to form a metapopulation structure in the habitat that remains and that stochastic extinction forces alone will be sufficient to severely reduce the regional distribution of this species. This study demonstrates that mtDNA is a useful tool for detecting contemporary population phenomena and can provide qualitative information of practical importance to wildlife managers.     

Population differentiation in the marginal populations of the great tit (Paridae: Parus major)

The major subspecies group of the great tit, Parus major , has experienced demographic and spatial expansions during the last century in several sites at the edges of its distribution range. These expansions, although temporarily very even, have resulted in dissimilar patterns of molecular diversity. Populations locating at regions of contact to other subspecies groups (in Amur, Kirghizia–Kazakhstan, and Iran) show divergence from central population by nuclear and mitochondrial markers. In Amur, gene flow from minor group could be detected based on the existence of private minor alleles in the major population. In Kirghizia and Kazakhstan, the bokharensis and major groups share almost all the microsatellite alleles detected though frequencies differ. In Iran, three geographically close populations are distinct according to the mitochondrial sequences but also indications of present or recent admixture is detected. Populations, which have expanded to regions previously unoccupied by the species (northern UK and Finland), show divergence only by one of the markers. The variability in molecular differentiation may be due to dissimilar expansions, depending on whether the colonized regions have previously been occupied by another subspecies or not, on the amount of colonizing birds, and on the amount of past and present gene flow.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 90 , 201–210.     

Spatial dynamics of colony interactions in young populations of the fire ant Solenopsis invicta

Newly founded colonies of the fire ant Solenopsis invicta compete intensely by brood raids, which result in a rapid reduction of colony density. Experimental plantings of colonies and analyses of sequential maps were used to examine the importance of spatial pattern in the dynamics of young populations. Colony positions were initially clumped in naturally founded cohorts, but were regular in most mature populations. Incipient colonies planted in clumped patterns were more likely to engage in brood raids than colonies planted in regular hexagonal patterns at the same average density. However, contrary to what would be expected if local crowding increased mortality, no significant increases in spatial regularity were observed during brood raiding either in the experimentally planted populations or in a natural population of more than 1200 incipient colonies. These results show that it may be difficult to infer the degree of past or current competition by passive analysis of spatial data even when field experiments show that the probability of mortality depends on local spacing.     

Genetic structure of populations of the closely related species of Melanopsis (Gastropoda: Cerithiacea) in Israel

Population genetic structure in the species of Melanopsis were studied by means of cellulose acetate gel allozyme electrophoresis, on 26 Melanopsis populations from Israel: six of Melanopsis buccinoidea Olivier, 1801, eight of Melanopsis saulcyi Bourguignat, 1853, one of Melanopsis meiostoma Heller et Sivan, 2000 , 11 of Melanopsis costata Olivier, 1804, represented by two subspecies: M. costata costata Olivier, 1804 and M. costata jordanica Roth, 1839. 14 loci (nine polymorphic) were scorable: Aat, Alp, Est-1, Est-2, Gpi, Hbdh, Idh-1, Idh-2, Iddh, Mdh, Mdhp, Mpi, Pgdh, Pgm. Gametic disequilibrium was postulated. D-statistics was computed, indicating limited migration, not epistatic selection as the source of disequilibrium. Exact multilocus and multipopulation tests showed a statistically significant heterozygote deficit in 18 populations and seven polymorphic loci. Inbreeding, Wahlund's effect and codominant mode of selection were postulated as causing homozygote excess. Mantel test indicated a statistically significant association between the pairwise θ and geographic distance, and no association between Nm and the geographic distance. The mean gene flow estimates Nm, derived from either θ or private alleles technique, were consistent. Hierarchical F-statistics showed slight differences between the taxa. The process of speciation within the genus seems not yet completed.     

Hierarchical analysis of population genetic structure in the monogynous ant Cataglyphis cursor using microsatellite and mitochondrial DNA markers

Despite having winged queens, female dispersal in the monogynous ant Cataglyphis cursor is likely to be restricted because colonies reproduce by fission. We investigated the pattern of population genetic structure of this species using eight microsatellite markers and a mitochondrial DNA (mtDNA) sequence, in order to examine the extent of female and nuclear gene flow in two types of habitat. Sampling was carried out at a large spatial scale (16 sites from 2.5 to 120 km apart) as well as at a fine spatial scale (two 4.5-km transects, one in each habitat type). The strong spatial clustering of mtDNA observed at the fine spatial scale strongly supported a restricted effective female dispersal. In agreement, patterns of the mtDNA haplotypes observed at large and fine spatial scales suggested that new sites are colonized by nearby sites. Isolation by distance and significant nuclear genetic structure have been detected at all the spatial scales investigated. The level of local genetic differentiation for mitochondrial marker was 15 times higher than for the nuclear markers, suggesting differences in dispersal pattern between the two sexes. However, male gene flow was not sufficient to prevent significant nuclear genetic differentiation even at short distances (500 m). Isolation-by-distance patterns differed between the two habitat types, with a linear decrease of genetic similarities with distance observed only in the more continuous of the two habitats. Finally, despite these low dispersal capacities and the potential use of parthenogenesis to produce new queens, no signs of reduction of nuclear genetic diversity was detected in C. cursor populations.     

Population structure and gene flow of the Atlantic walrus (Odobenus rosmarus rosmarus) in the eastern Atlantic Arctic based on mitochondrial DNA and microsatellite variation

The population structure of the Atlantic walrus, Odobenus rosmarus rosmarus , was studied using 11 polymorphic microsatellites and restriction fragment length polymorphism detected in the NADH-dehydrogenase ND1, ND2 and ND3/4 segments in mtDNA. A total of 105 walrus samples were analysed from northwest (NW) Greenland, east (E) Greenland, Svalbard and Franz Joseph Land. Two of the 10 haplotypes detected in the four samples were diagnostic for the NW Greenland sample, which implied that the group of walruses in this area is evolutionary distinct from walruses in the other three areas. One individual sampled in E Greenland exhibited a Pacific haplotype, which proved a connection between the Pacific walrus and walruses in eastern Greenland. The Franz Joseph Land, Svalbard and E Greenland samples shared the most common haplotype, indicating very little differentiation at the mtDNA level. Gene flow ( Nm ) estimates among the four areas indicated a very restricted exchange of female genes between NW Greenland and the more eastern Atlantic Arctic samples, and a closer relationship between the three samples composing the eastern Atlantic Arctic. The genetic variation at 11 polymorphic microsatellite loci grouped individuals into three populations, NW Greenland, E Greenland and a common Franz Joseph Land–Svalbard population, which were connected by moderate gene flow.     

Genetic structure and history of populations of the deep-sea fish Helicolenus dactylopterus (Delaroche, 1809) inferred from mtDNA sequence analysis

Helicolenus dactylopterus is an Atlantic benthopelagic fish species inhabiting high-energy habitats on continental slopes, seamounts and islands. Partial sequences of the mitochondrial control region (D-loop) and cytochrome b (cyt b) were used to test the hypothesis that H. dactylopterus disperses between continental margin, island and seamount habitats on intraregional, regional and oceanic scales in the North Atlantic. Individuals were collected from five different geographical areas: Azores, Madeira, Portugal (Peniche), Cape Verde and the northwest Atlantic. D-loop (415 bp) and cyt b (423 bp) regions were partially sequenced for 208 and 212 individuals, respectively. Analysis of variation among mitochondrial DNA sequences based on pairwise F-statistics and AMOVA demonstrated marked genetic differentiation between populations in different geographical regions specifically the Mid-Atlantic Ridge (Azores)/northeast Atlantic (Portugal, Madeira) compared to populations around the Cape Verde Islands and in the northwest Atlantic. Some evidence of intraregional genetic differentiation between populations was found. Minimum-spanning network analysis revealed star-shaped patterns suggesting that populations had undergone expansion following bottlenecks and/or they have been colonized by jump dispersal events across large geographical distances along pathways of major ocean currents. Mismatch distribution analysis indicated that Azores and northwest Atlantic populations fitted a model of historical population expansion following a bottleneck/founder event estimated to be between 0.64 and 1.2 million years ago (Ma).     

Population genetics and history of the introduced fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), in Australia

Abstract  The red imported fire ant, Solenopsis invicta , a damaging invasive pest, was discovered in February 2001 in Brisbane, Australia at two sites, Fisherman Islands and suburban Richlands-Wacol. Using four microsatellite loci and the protein marker Gp-9 , we compared the two infestations with each other, and with potential source populations in North and South America to better understand the history of their introduction to Brisbane. Based on an analysis of molecular variance, as well as a maximum likelihood tree of colonies from the two Australian sites, we found that the two sites were genetically distinct and were almost certainly introduced separately. All of the colonies at Fisherman Islands were monogynous, headed by a single queen, while the Richlands-Wacol site had a mixture of single-queen monogynous and multiple-queen polygynous colonies. However, the monogynous and polygynous colonies at the Richlands-Wacol site were not genetically distinct from each other, and probably constitute a single, mixed introduction. Based on allele frequencies at the microsatellite loci, and Gp-9 , both Australian infestations were more similar to North American populations than to South American, though the Fisherman Islands infestation was intermediate, making it difficult to assign. Thus, there has been one introduction from either a North or South American monogynous population at Fisherman Islands, and one introduction from a mixed monogynous/polygynous North American population at Richlands-Wacol. These findings have implications for the control of the current infestations, as well as for the quarantine regulations necessary to prevent additional introductions to Australia.     

Genetic Diversity and Spatial Genetic Structure of an Epiphytic Bromeliad in Costa Rican Montane Secondary Forest Patches

Information on genetic variation and its distribution in tropical plant populations relies mainly on studies of ground‐rooted species, while genetic information of epiphytic plants is still limited. Particularly, the effect of forest successional condition on genetic diversity and structure of epiphytes is scanty in the literature. We evaluated the genetic variation and spatial genetic structure of the epiphytic bromeliad Guzmania monostachia (Bromeliaceae, Tillandsioideae) in montane secondary forest patches in Costa Rica. The sampling design included plants on the same trees (i.e., populations), populations within forest patches and patches within secondary forest at two different successional stages (early vs. mid‐succession). Six microsatellites revealed low levels of population genetic variation (A = 2.06, A_E = 1.61, H_E = 0.348), a marked deficiency of heterozygotes (H_O = 0.031) and high inbreeding (f = 0.908). Genetic differentiation was negligible among populations within the same forest patch, but moderate (G_ST = 0.123 ± 0.043) among forest patches. Genetic relatedness between individuals was significantly higher for plants located within the same forest patch and separated by <60 m and decreased as distance between plants increased, becoming significantly negative at distances >400 m. An analysis of molecular variance (AMOVA) showed significant genetic variation between forest patches, but non‐significant variation between successional stages. The selfing breeding system and limited seed dispersal capabilities in G. monostachia could explain the observed levels and partitioning of genetic diversity at this geographic scale. However, these results also suggest that forest fragmentation is likely to influence the degree of local genetic structuring of epiphytic plants by limiting gene flow.     

Genetic structure of kestrel populations and colonization of the Cape Verde archipelago

Genetic diversity and population structure were studied in eight populations of the kestrel Falco tinnunculus to identify the genetic consequences of spatial distribution and to infer the colonization patterns of the Cape Verde archipelago. We studied genetic differentiation and gene flow among seven island populations and one mainland population using nine microsatellite loci. Within the archipelago, differentiation was strong and genetic diversity and heterozygosity were low but variable among populations. Two subspecies F. tinnunculus neglectus on the northwestern islands and F. tinnunculus alexandri on all the other islands were identified as genetically distinct units. F. t. alexandri could be further separated into two groups on eastern and southern islands. Populations are probably founded by birds originating from the mainland. Immigration is more likely to the eastern and southern populations, whereas the northwestern islands with the lowest genetic diversity and highest differentiation are likely to exhibit fewer founding events by immigrants. The number of founding events on each island may depend not only on geographical distance to neighbouring populations, but also on directional immigration due to the northeastern trade winds. This may explain differences in genetic differentiation and diversity between populations and subspecies and may enable allopatric speciation.