Perttu Seppä

GENE FLOW AND POPULATION VISCOSITY IN MYRMICA ANTS

The amount of gene flow and population viscosity were studied in two red ant species, Myrmica ruginodis and M. rubra. Differentiation between populations at the local scale (within the dispersal distance of individuals) and between localities further apart was estimated using Wrights F-statistics. Population viscosity was described using spatial autocorrelation of allele frequencies of nests. The differentiation patterns in the two species are clearly distinct, FST values being an order of magnitude larger in M. rubra (M. rubra: FST = 0.205 between sites within localities and 0.199 between localities; M. ruginodis: FST = 0.027 and 0.014 between sites within localities, 0.009 between localities). Thus, assuming the island model, much less gene flow occurs between neighbouring M. rubra than M. ruginodis sites. Allele frequencies of nests close to each other are positively autocorrelated in both species. This suggests that new nests are commonly founded close to the mother nest, most probably by division of existing nests. M. rubra forms large multinest societies, which dominate substantial areas, whereas such multinest colonies are rare in M. ruginodis.

Sociogenetic organization of the ants Myrmica ruginodis and Myrmica lobicornis: Number, relatedness and longevity of reproducing individuals

The number and relationships of reproducing individuals create the observed genetic heterogeneity within a social insect colony. These are referred to as sociogenetic organization and were studied in the red ants M. ruginodis and M. lobicornis. Direct observations of the queen numbers were obtained by excavating colonies. The effective number of reproducing individuals was estimated from genetic relatedness based on genotype frequency data. Sociogenetic organization of colonies of both species is simple. The number of queens is low, single mating of queens is the rule and queen to queen variation in worker production is minor. The important variables of sociogenetic organization are the number and relatedness of coexisting queens in polygynous colonies. Queen nestmates are related on average by 0.405 in polygynous colonies of M. ruginodis, showing that colonies recruit their own daughters as new reproductives. The distribution of queen number in M. ruginodis indicates that the study population contains both microgyna and macrogyna types of the species. The large proportion of colonies where the resident queen(s) is not the mother of the workers shows that the average life span of a queen is short and colonies are serially polygynous.

Low paternity in the hornet Vespa crabro indicates that multiple mating by queens is derived in vespine wasps

Abstract Queen mating frequency was studied in the European hornet, Vespa crabro, by analyzing four DNA microsatellite loci in 20 workers from each of 14 nests. Queens were found to be predominantly singly mated (9/14), although double (4/14) and triple mating (1/14) also occurred. For most multiply mated queens, paternity was significantly biased with the majority male fathering on average 80% of the female offspring. The population-wide effective mating frequency was therefore low (1.11), and sister-sister relatedness high (0.701 ± 0.023 SE). Low effective mating frequency in Vespa, in combination with data from other vespines, suggests that high paternity frequency is derived in the group. Some problems with the non-detection of fathers, where the queen was not sampled or shared alleles with males, are analyzed.

Genetic structure of island populations of the anurans Rana temporaria and Bufo bufo

Amphibians have traditionally been considered to have low dispersal ability and they have become a model for studies on the effects of man-made habitat fragmentation on genetic variation and population differentiation. This study examined the genetic population structure in the common frog (Rana temporaria) and the common toad (Bufo bufo) in an archipelago of the northern Baltic Sea. Heterozygosity was not correlated with distance of the island from the mainland nor, in R. temporaria, with effective population size based on census estimates. Generally, no inbreeding was detected in island populations. The overall differentiation among islands was weak, but the FST values were significantly larger in R. temporaria (FST = 0.068) than in B. bufo (FST = 0.019). Most of the differentiation was a result of differences among groups of islands, differentiation within them playing a minor role. Thus, assuming Wright’s island model of migration, gene flow was rather high among closely located islands, but longer distances seemed to form a slight dispersal barrier for R. temporaria. Strong gene flow within the study area was confirmed by lack of isolation by distance. The estimated effective population sizes in R. temporaria were small, the average being 32 breeding females per island. The results indicate that gene flow between island populations across the matrix of open, brackish-water sea is extensive and suggest that the anurans are well able to disperse in this natural metapopulation system.

Conservation genetics of the wood ant, Formica lugubris , in a fragmented landscape

Various intrinsic factors connected to the special features of sociality influence the persistence of social insect populations, including low effective population size, reduced amount of genetic variation easily leading to inbreeding depression, and spatially structured populations. In this work, we studied an isolated, small and fragmented population system of the red wood ant Formica lugubris, and evaluated the impact of social and genetic population structure on the persistence and conservation of the populations. The effective population size was large in our study population because all nests were polygynous. As a result, and despite the apparent isolation, the amount of nuclear genetic variability was similar to that in a nonisolated population system. Lack of inbreeding, as well as a high level of variability, indirectly suggests that this population does not suffer from inbreeding depression. The spatial distribution of genetic variation between local populations suggests intensive, but strongly male‐biased, nuclear gene flow. Thus, the persistence of this population system does not seem to be threatened by any immediate social or genetic factor, but colonization of new habitat patches may be difficult because of restricted female dispersal.

Can cuticular lipids provide sufficient information for within-colony nepotism in wasps?

Inclusive fitness theory predicts that members of non–clonal societies will gain by directing altruistic acts towards their closest relatives. Multiple mating by queens and multiple queens creates distinct full–sister groups in many hymenopteran societies within which nepotism might occur. However, the weight of empirical data suggests that nepotism within full–sister groups is absent. It has been suggested that a lack of reliable recognition markers is responsible. In this paper, we investigated whether epicuticular lipids could provide reliable cues for intracolony kin recognition in two species of social wasps, the paper wasp Polistes dominulus and the hornet Vespa crabro. Epicuticular lipids have previously been shown to be central to kin recognition at the nest level, making them excellent candidates for within–nest discrimination. We genotyped individuals using DNA microsatellites and analysed surface chemistry by gas chromatography–mass spectrometry. We find that in both species epicuticular lipids typically could provide enough information to distinguish related nest–mates from unrelated nest–mates, a difference that occurs in colonies with multiple queens. However, in V. crabro, where colonies may be composed by different patrilines, information for discrimination between full sisters and half–sisters is weaker and prone to errors. Our data suggest that epicuticular lipids at best provide reliable information for intracolony nepotistic discrimination in multiple–queen colonies composed of unrelated lines.

COEXISTENCE OF THE SOCIAL TYPES: GENETIC POPULATION STRUCTURE IN THE ANT FORMICA EXSECTA

Abstract —The ant Formica exsecta has two types of colonies that exist in sympatry but usually as separate subpopulations: colonies with simple social organization and single queens (M type) or colonial networks with multipe queens (P type). We used both nuclear (DNA microsatellites) and mitochondrial markers to study the taransition between the social types, and the contribution of males nd femeles in gene flow within and between the types. Our results showed that the social types had different spatial genetic stractures. The M subpoplations formed a fairly uniform population, whereas the P subpopulations were, on average, more differentiated from each other than from the nearby M bubpoplations and could have been locally established from the M‐type colonies, followed by philopatric behavior and restrcted emigration of females. Thus, the relationship between the two social types resembles that of souce (M type) and sink (P type) populations. The comparison of mitochondrial (Φst) and nuclear (Fst) differentiation indicates that the dispersal rate of males is four to five times larger than that of females both among the P‐type subpopulations and between the social types. Our results suggest that evolution toward complex social organization can have an important effect on genetic population structure through changes in dispersal behavior associated with different socigenetic orgamizations.

Sociogenetic organization of the red ant Myrmica rubra

Abstract Knowledge of the sociogenetic organization determining the kin structure of social insect colonies is the basis for understanding the evolution of insect sociality. Kin structure is determined by the number and relatedness of queens and males reproducing in the colonies, and partitioning of reproduction among them. This study shows extreme flexibility in these traits in the facultatively polygynous red ant Myrmica rubra. Relatedness among worker nestmates varied from 0 to 0.82. The most important reason for this variation was the extensive variation in the queen number among populations. Most populations were moderately or highly polygynous resulting in low relatedness among worker nestmates, but effectively monogynous populations were also found. Polygynous populations also often tend to be polydomous, which is another reason for low relatedness. Coexisting queens were positively related in two populations out of five and relatedness was usually similar among workers in the same colonies. Due to the polydomous colony organization and short life span of queens, it was not possible to conclusively determine the importance of unequal reproduction among coexisting queens, but it did not seem to be important in determining the relatedness among worker nestmates. The estimates of the mating frequency by queens remained ambiguous, which may be due to variation among populations. In some populations relatedness among worker nestmates was high, suggesting monogyny and single mating by queens, but in single-queen laboratory nests relatedness among the worker offspring was lower, suggesting that multiple mating was common. The data on males were sparse, but indicated sperm precedence and no relatedness among males breeding in the same colony. A comparison of social organizations and habitat requirements of M. rubra and closely related M. ruginodis suggested that habitat longevity and patchiness may be important ecological factors promoting polygyny in Myrmica.

Worker reproduction in ants — a genetic analysis

Workers of social insects may enhance their inclusive fitness by laying unfertilized eggs that develop into males. In particular, workers may gain from rearing worker-produced males if their average relatedness to them exceeds their relatedness to queen-produced males. These relatedness values depend both on the queen mating frequency and on the number and relatedness of nestmate queens. We examined the occurrence of worker reproduction in field colonies of four ant species of the genera Formica and Myrmica. Based on relatedness arguments alone, worker reproduction was expected in all species because of low queen mating frequency, or low effective queen numbers. Nevertheless, genotype matching of workers and males showed that worker reproduction was absent or rare in two of the three Formica species studied here. In M. ruginodis, queens may have been the sisters of the workers in many cases, which means that workers of this species may regularly rear nephews. In the three species in which worker reproduction was not found, workers bias colony sex ratios to enhance their inclusive fitness. We therefore hypothesize that sex ratio biasing and male production may be mutually exclusive strategies for workers.

Absence of within–colony kin discrimination in behavioural interactions of swarm–founding wasps

Within–colony kin discrimination has not been demonstrated conclusively for any social insect, perhaps partly because highly polymorphic genetic markers necessary to assess within–colony relatednesses have only recently become available. We use microsatellite loci to investigate within–colony kin discrimination in behavioural interactions in the neotropical multiple–queen wasp, Parachartergus colobopterus. Within–colony kin discrimination would be particularly advantageous in this species since average genetic relatedness among colony members overall is low (0.32 =/– 0.06), compared to the relatedness value between full sisters of 0.75. Using seven colonies of individually marked females, we recorded behavioural interactions that were cooperative (222 grooming, 2438 feeding), aggressive (511 body or wing biting, 240 mandible biting) or neutral (1676 antennating). We expected cooperative behaviours to favour closer kin and aggressive behaviours to be directed towards more distant kin, but found that none of the behaviours we investigated showed discrimination on the basis of relatedness. We could have detected a difference in relatedness values of as little as between 0.03 and 0.12, depending on the behaviour being analysed. Thus, we found no evidence for kin discrimination in within–colony behaviour in this species.