David J. C. Fletcher

Kin recognition in animals.

Kin recognition - the differential treatment of kin and non-kin by an individual within a species - is one of the most interesting and quickly developing topics in modern biology. Researchers have been astonished and fascinated to discover the sophistication and subtlety of the ways individuals in even simple species, distinguish not only kin from non-kin, but also siblings, half-siblings and cousins. In many cases these forms of social behaviour appear to enhance the survival of the group rather than the individual and it is a matter of considerable sociological interest to establish how far such altruistic behaviour is the result of genetically determined traits, and how far it is learnt. Research on this subject is necessarily widely spread across many taxa and many disciplines. This edited collection of papers from academics gives an overview of the whole field, presenting (in some cases, original) research on all the major animal groups which have been studied. It pays particular attention to general methodology and to the specific methods employed in experimental work, but its main strength concerns its treatment of concepts. These are clearly presented and are evaluated from the different points of view of various contributors. Some concepts, especially that of nepotism (the favouring of kin) emerge with greatly enhanced significance.

Comparative study of genetic and social structure in two forms of the fire ant Solenopsis invicta (Hymenoptera: Formicidae)

SummaryThe genetic and social structures of polygyne and monogyne forms of the fire ant, Solenopsis invicta, are investigated in a comparative manner using allozyme data from two polymorphic loci. Foundress queens of the monogyne form are signly inseminated and appear to produce all males present in the colony during the major summer mating flights. The average regression coefficient of relatedness (b) among female nestmates of the monogyne form is 0.714 (Fig. 2), statistically indistinguishable from the pedigree coefficient of relatedness (G) of 0.75. We suggest that the evolution of obligate worker sterility in Solenopsis is associated with this high relatedness between workers and the queens they rear. Functional queens in polygyne nests also are singly inseminated and are no more closely related to nestmate queens than to other queens (within-nest b=0). Within-nest relatedness of workers in the polygyne population is similarly low (Fig. 2). Both the monogyne and polygyne populations from northcentral Georgia are in Hardy-Weinberg equilibrium at both allozyme loci and we found no evidence of significant population subdividion or inbreeding in the polygyne population. These results do not support the view that kin selection has promoted the evolution of polygyny in North American S. invicta. Rather, mutualism appears to be the most likely selective factor mediating queen associations inthis ant.

Genetic origin of male diploidy in the fire ant, Solenopsis invicta (Hymenoptera : Formicidae), and its evolutionary significance

Male diploidy was studied in natural populations of the fire ant, Solenopsis invicta, in order to find an explanation from population genetics for the apparently common occurrence of this phenomenon in some North American populations. The association of male diploidy with polygyne (more than one queen per colony) populations in this species led us to expect that the two phenomena are causally linked. We proposed three hypotheses, based on current knowledge of sex‐determining mechanisms in the Hymenoptera, to explain the loss of genetic diversity associated with high rates of diploid male production in S. invicta: a) allelic diversity was reduced during colonization of North America by a small founder group; b) allelic diversity was reduced during subsequent subfounding of the several polygyne populations; and c) genetic structuring of polygyne populations due to local inbreeding caused reduced allelic diversity and/or increased homozygosity.

Diploid male production — a significant colony mortality factor in the fire ant Solenopsis invicta (Hymenoptera: Formicidae)

SummaryTwo forms of the fire ant, Solenopsis invicta, occur in North America; the monogyne form has colonies with a single functional queen while the polygyne form has colonies containing many functional queens. Field surveys indicate that diploid males are common in natural populations of the polygyne form but absent from monogyne populations, in contrast to laboratory data showing that similar frequencies of queens producing such males occur in the two types of populations. Our results show that mature monogyne colonies with adopted queens rear diploid males in the laboratory, so it is unlikely that the absence of these males from monogyne colonies in the field is due to discrimination against them by monogyne workers. On the other hand, incipient monogyne colonies that produce diploid males exhibit significantly higher mortality and significantly slower rates of growth (Figs. 1–3) than colonies producing workers only. These results suggest that the observed distribution of male diploidy in S. invicta can be explained by differential mortality of diploid male producing colonies of the two forms, with such colonies of the monogyne form experiencing 100% mortality early in development. The mortality differences due to this factor are shown to be related to the different social structures and modes of colony founding characterizing the two forms.

BIOCHEMICAL PHENOTYPIC AND GENETIC STUDIES OF TWO INTRODUCED FIRE ANTS AND THEIR HYBRID (HYMENOPTERA: FORMICIDAE)

Two introduced fire ants, Solenopsis invicta and S. richteri, and their hybrid were studied using phenotypic markers from gas chromatographic analysis of hydrocarbons and venom alkaloids, as well as genetic markers from enzyme electrophoresis. Both methods show that extensive gene introgression is occurring over a distance of at least 120 km at the contact zone between the two forms in eastern Mississippi. Genetic analyses suggest that the hybrid population does not depart dramatically from panmixia. Also, recombinant genotypes predominate in the hybrid zone, indicating that F1 hybrids are viable. Allele frequency clines through the hybrid zone are apparent for four polymorphic loci. Data sets generated by the chromatographic and electrophoretic methods are highly concordant in that they differentiate completely between the two forms and agree in designating colonies from the contact zone as hybrid or parental in a high proportion (90%) of cases. The two methods can serve as complementary tools for studying closely related but genetically distinct populations in this, and perhaps other, groups of insects.

Regulation of Queen Number by Workers in Colonies of Social Insects

Experiments with fire ants suggest that queen pheromones act quantitatively in the regulation of queen number in colonies of social insects. Specific mechanisms probably include recognition by workers of unique quantitative blends of pheromones produced by queens, and quantitative effects of pheromones acting at the level of the colony on workers and at the level of the individual on queens. Several aspects of this quantitative hypothesis of pheromone action were tested.

On the relationship between queen number and fecundity in polygyne colonies of the fire ant Solenopsis invicta

ABSTRACT. The reproductive status of individual queens in relation to the number of queens in polygyne colonies of Solenopsis invicta Buren was investigated under both field and laboratory conditions. In field colonies, the weight of individual queens is inversely related to the number of resident queens present. The weight of a queen is positively correlated with fecundity, as measured by the number of eggs laid in 5 h in isolation. Thus increasing queen number has a strong negative effect on the fecundity of individual queens in these colonies. A similar relationship was established in standardized laboratory colonies using queens from a single experimental population. Colony size by itself had no significant effect on queen weight in either the field or laboratory colonies, suggesting that the inverse relationship between queen number and fecundity results from mutual inhibition among queens, possibly involving pheromones, rather than reduced nutrition through lower worker/queen ratios. While uninseminated queens can comprise up to 50% of the functional (i.e. egglaying) queens in polygyne colonies of this ant, they tend to be less fecund than their inseminated nestmates; uninseminated queens within a given colony generally weigh 5–25% less.

The inhibitory pheromone of queen fire ants: effects of disinhibition on dealation and oviposition by virgin queens

Summary1.Virgin queen fire ants,Solenopsis invicta Buren, that overwintered in parental nests participated in mating flights in early spring, and some were shown to have been laying eggs in queenright colonies before leaving. Virgin queens reared in spring had immature oocytes in their ovarioles when they left on mating flights.2.When released from pheromonal queen influence by isolation, 98% of both overwintered and spring-reared virgin queens oviposited and exhibited colony founding behavior, but only 73% dealated. The alary muscles of all of them histolyzed, however, whether or not they dealated. The eggs laid by both the overwintered and springreared virgin queens were larger than those laid by physogastric mated queens, and less than 1% were viable.3.When spring-reared virgin queens were disinhibited by dividing colonies into queenright and queenless halves, some virgin queens in the queenless halves dealated and oviposited. A frequency distribution of the sizes of eggs laid by these virgin replacement queens was bimodal. The larger eggs were similar in size to those laid by spring-reared virgin queens in isolation and less than 1% were embryonated. They served a trophic function. The smaller eggs were not as small as those laid by mated queens, but the majority were embryonated. The number of eggs laid by virgin replacement queens in 5 h was positively correlated with the maximum number of oocytes/ovariole. The size of eggs laid by virgin and mated queens was a function of the oviposition rate.4.The results demonstrate that the inhibitory pheromone of queen fire ants acts directly on virgin queens and that it prevents muscle histolysis and rapid oogenesis. The former hypothesis that the pheromone acts mainly on workers and prevents them from causing virgin queens to dealate is, therefore, disproved.5.A new working hypothesis consistent with the experimental results and with previously published information is that a single queen pheromone regulates the secretion, and titer, of juvenile hormone.

Effect of queen number on the production of sexuals in natural populations of the fire ant, Solenopsis invicta

ABSTRACT. To determine whether the production of sexuals in the fire ant, Solenopsis invicta Buren, is related to queen number in the field, the numbers of sexuals in monogyne (M) and polygyne (P) colonies of this species were compared. Large colonies (n=25 M and 25 P) were collected at random during spring, summer and fall when sexuals are actively produced, and the numbers of sexuals present were counted. The numbers of alates plus sexual pupae and larvae in M colonies far exceeded those in P colonies in each season. Significant differences between M and P colonies were also found when the numbers of alates and sexual immatures (pupae + larvae) were compared separately in each season. In addition, M colonies contained significantly more female sexuals (alates + pupae) than did P colonies in the summer and fall, and significantly more males in all three seasons. The negative relationship between queen number and number of sexuals provides evidence that queen control over the production of sexuals, previously established in laboratory experiments, also occurs under natural conditions.

Evidence of pheromonal queen control over the production of male and female sexuals in the fire ant,Solenopsis invicta

SummaryTwo hypotheses that could explain social regulation of the production of sexuals inSolenopsis invicta were investigated: (1) differences in worker/larva ratios; and (2) pheromonal regulation by queens. Small laboratory units (fragments of multiple-queen, i.e. polygyne, colonies) were found to be capable of producing sexuals and to be sensitive in this regard to differences in queen number; the presence of even a single queen inhibited the numbers of sexuals (male and female) produced in comparison with queenless controls. To determine whether differences in worker/larva ratios were involved in this inhibition, a greater number of eggs than could be laid by a single queen was added daily to queenless units. These units produced more sexuals than similar units containing one queen, but did not differ from queenless controls which did not receive eggs. These results rule out the possibility that differences in worker/ larva ratios significantly affected the production of sexuals in such experimental units. They also exclude the possibility that chemical cues were transmitted by queens via their eggs. To test the hypothesis that queen pheromones were involved, freshly killed corpses of functional (egg-laying) queens were added daily to queenless units. These effectively inhibited the production of sexuals, although not as effectively as living queens. On the other hand, corpses of alate virgin (non-egg-laying) queens did not inhibit the production of sexuals. The addition of live queens to previously queenless units in which large sexual larvae had developed resulted in the execution of most of these larvae by workers, indicating that queen control over the production of sexuals can act retroactively after larvae are sexualized. A similar but less pronounced result was obtained by the addition of functional queen corpses. These results provide evidence that functional queens exert control over the production of sexuals inS. invicta through pheromones that influence the behavior of workers toward both male and female larvae.