Sex ratio and gamete size across eastern North America in Dictyostelium discoideum,a social amoeba with three sexes

Theory indicates that numbers of mating types should tend towards infinity or remain at two. The social amoeba, Dictyostelium discoideum, however, has three mating types. It is therefore a mystery how this species has broken the threshold of two mating types, but has not increased towards a much higher number. Frequency‐dependent selection on rare types in combination with isogamy, a form of reproduction involving gametes similar in size, could explain the evolution of multiple mating types in this system. Other factors, such as drift, may be preventing the evolution of more than three. We first looked for evidence of isogamy by measuring gamete size associated with each type. We found no evidence of size dissimilarities between gametes. We then looked for evidence of balancing selection, by examining mating type distributions in natural populations and comparing genetic differentiation at the mating type locus to that at more neutral loci. We found that mating type frequency varied among the three populations we examined, with only one of the three showing an even sex ratio, which does not support balancing selection. However, we found more population structure at neutral loci than the mating type locus, suggesting that the three mating types are indeed maintained at intermediate frequencies by balancing selection. Overall, the data are consistent with balancing selection acting on D. discoideum mating types, but with a sufficiently weak rare sex advantage to allow for drift, a potential explanation for why these amoebae have only three mating types.     

Queens, not workers, produce the males in the stingless bee Schwarziana quadripunctata quadripunctata

Most stingless bee colonies have one singly mated queen, resulting in a potential conflict between workers and queen over male production, because workers are more closely related to the sons of other workers than they are to the queen's sons. Furthermore, workers in the majority of stingless bee species have ovarian development, can produce haploid eggs, and show apparently agonistic behaviour towards their queen, suggesting a real conflict. We investigated whether genetic conflict over male production resulted in reproductive and behavioural conflict in Schwarziana q. quadripunctata. DNA microsatellite loci showed that, even though workers are more related to other workers' sons than to queen's sons, it is the queen who produces the males. Behavioural interactions between workers and their queen were not uniformly more aggressive during male production than at times with only worker production, although some differences might have been obscured by the fact that food supply was greater during male production. The potential conflict over male production inS. q. quadripunctata seems not to lead to an observable conflict between the workers and their queen. Workers might refrain from reproduction because of the costs involved for the colony or because of queen control.     

Swarm-founding in the polistine wasps: the importance of finding many microsatellite loci in studies of adaptation

We developed 52 microsatellite loci for the wasp, Polybioides tabidus, for the purpose of studying the evolution and inclusive fitness consequences of swarm-founding. The large number of loci is important for three reasons that may apply to many other systems. Heterozygosity was low in our target species, yet we found enough polymorphic loci for accurate kinship studies in this species. Many monomorphic loci were polymorphic in other polistine wasps, making comparative studies possible. Finally, enough loci amplified over a broad range of species to add a historical dimension. We sequenced six loci in other polistine wasps and used the flanking sequences to construct a phylogeny. Based on this phylogeny, we infer that swarm-founding has evolved independently three times in the polistine wasps.     

The costs and benefits of being a chimera

Most multicellular organisms are uniclonal. This is hypothesized to be because uniclonal organisms function better than chimeras (non-clonal organisms), owing to reduced levels of internal genetic conflict. We tested this idea using the social amoeba or slime mold Dictyostelium discoideum. When starving, the normally solitary amoebae aggregate to form a differentiated multicellular slug that migrates towards light and forms a fruiting body, facilitating the dispersal of spores. We added 10(7) amoebae to Petri plates containing 1, 2, 5 or 10 clones mixed together. We found an intrinsic cost to chimerism: chimeric slugs moved significantly less far than uniclonal slugs of the same size. However, in nature, joining with other clones to form a chimera should increase slug size, and larger slugs travel further. We incorporated this size effect into a second experiment by giving chimeras more cells than single clones (single clones had 10(6) cells, two-clone chimeras had 2 x 10(6) cells and so on). The uniclonal treatments then simulated a clone in a mixture that refuses to form chimeras. In this experiment, chimeras moved significantly further than the uniclonal slugs, in spite of the intrinsic cost. Thus, chimerism is costly, which may be why it evolves so seldom, but in D. discoideum the benefits of large size appear to compensate.     

Social amoebae mating types do not invest unequally in sexual offspring

Unequal investment by different sexes in their progeny is common and includes differential investment in the zygote and differential care of the young. The social amoeba Dictyostelium discoideum has a sexual stage in which isogamous cells of any two of the three mating types fuse to form a zygote which then attracts hundreds of other cells to the macrocyst. The latter cells are cannibalized and so make no genetic contribution to reproduction. Previous literature suggests that this sacrifice may be induced in cells of one mating type by cells of another, resulting in a higher than expected production of macrocysts when the inducing type is rare and giving a reproductive advantage to this social cheat. We tested this hypothesis in eight trios of field‐collected clones of each of the three D. discoideum mating types by measuring macrocyst production at different pairwise frequencies. We found evidence that supported differential contribution in only two of the 24 clone pairs, so this pattern is rare and clone‐specific. In general, we did not reject the hypothesis that the mating types contribute cells relative to their proportion in the population. We also found a significant quadratic relationship between partner frequency and macrocyst production, suggesting that when one clone is rare, macrocyst production is limited by partner availability. We were also unable to replicate previous findings that macrocyst production could be induced in the absence of a compatible mating partner. Overall, mating type‐specific differential investment during sex is unlikely in microbial eukaryotes like D. discoideum.     

ESTIMATING RELATEDNESS USING GENETIC MARKERS

A new method is described for estimating genetic relatedness from genetic markers such as protein polymorphisms. It is based on Grafen's (1985) relatedness coefficient and is most easily interpreted in terms of identity by descent rather than as a genetic regression. It has several advantages over methods currently in use: it eliminates a downward bias for small sample sizes; it improves estimation of relatedness for subsets of population samples; and it allows estimation of relatedness for a single group or for a single pair of individuals. Individual estimates of relatedness tend to be highly variable but, in aggregate, can still be very useful as data for nonparametric tests. Such tests allow testing for differences in relatedness between two samples or for correlating individual relatedness values with another variable.     

An invitation to die: initiators of sociality in a social amoeba become selfish spores

Greater size and strength are common attributes of contest winners. Even in social insects with high cooperation, the right to reproduce falls to the well-fed queens rather than to poorly fed workers. In Dictyostelium discoideum, formerly solitary amoebae aggregate when faced with starvation, and some cells die to form a stalk which others ride up to reach a better location to sporulate. The first cells to starve have lower energy reserves than those that starve later, and previous studies have shown that the better-fed cells in a mix tend to form disproportionately more reproductive spores. Therefore, one might expect that the first cells to starve and initiate the social stage should act altruistically and form disproportionately more of the sterile stalk, thereby enticing other better-fed cells into joining the aggregate. This would resemble caste determination in social insects, where altruistic workers are typically fed less than reproductive queens. However, we show that the opposite result holds: the first cells to starve become reproductive spores, presumably by gearing up for competition and outcompeting late starvers to become prespore first. These findings pose the interesting question of why others would join selfish organizers.     

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.