Regulatory Mechanisms Underlying the Differentiation of Neotenic Reproductives in Termites: Partial Release From Arrested Development

Eusocial insects exhibit reproductive division of labor, in which only a part of colony members differentiates into reproductives. In termite colonies, the division of labors is performed among multiple types of individuals (i.e., castes), such as reproductives, workers, and soldiers to organize their society. Caste differentiation occurs according to extrinsic factors, such as social interactions, leading to developmental modifications during postembryonic development, and consequently, the caste ratio in a colony is appropriately coordinated. In particular, when the current reproductives die or become senescent, some immature individuals molt into supplementary reproductives, also known as “neotenics,” that take over the reproductive task in their natal colony. Neotenics exhibit variety of larval features, such as winglessness, and thus, immature individuals are suggested to differentiate by a partial release from arrested development, particularly in the reproductive organs. These neotenic features, which have long been assumed to develop via heterochronic regulation, provide us opportunities to understand the developmental mechanisms and evolutionary origin of the novel caste. This article overviews the accumulated data on the physiological and developmental mechanisms that regulate the neotenic differentiation in termites. Furthermore, the evolutionary trajectories leading to neotenic differentiation are discussed, namely the acquisition of a regulatory mechanism that enable the partial release from a developmentally arrested state.

Ant collective behavior is heritable and shaped by selection

Collective behaviors are widespread in nature and usually assumed to be strongly shaped by natural selection. However, the degree to which variation in collective behavior is heritable and has fitness consequences -- the two prerequisites for evolution by natural selection -- is largely unknown. We used a new pharaoh ant (Monomorium pharaonis) mapping population to estimate the heritability, genetic correlations, and fitness consequences of three collective behaviors (foraging, aggression, and exploration) as well as body size, sex ratio, and caste ratio. Heritability estimates for the collective behaviors were moderate, ranging from 0.17 to 0.32, but lower than our estimates for the heritability of caste ratio, sex ratio, and the body size of new workers, queens, and males. Moreover, variation among colonies in collective behaviors was phenotypically correlated, suggesting that selection may shape multiple colony collective behaviors simultaneously. Finally, we found evidence for directional selection that was similar in strength to estimates of selection in natural populations. Altogether, our study begins to elucidate the genetic architecture of collective behavior and is one of the first studies to demonstrate that it is shaped by selection.

The scaling of growth, reproduction and defense in colonies of Amazonian Termites

Phenotypes can evolve through life-history tradeoffs. Termites have been the first eusocial insects on Earth, prompting life history evolution at the colony level. Despite this, termite life-history allocation strategies are poorly known. Here, we addressed this issue using novel data on three common species from the diverse, yet understudied Amazonian termite fauna: Neocapritermes braziliensis, Labiotermes labralis and Anoplotermes banksi. Using Oster and Wilson’s optimal caste ratio theory and Higashi et al.’s termite caste allocation theory as frameworks, we assessed how termite colonies should invest in growth (immatures), reproduction (alates) and defense (soldiers) as they accumulate workers. We also examined whether soldier loss in soil-feeding Apicotermitinae (A. banksi) may have affected allocation strategies. We found that: (1) the scaling of immature number was isometric in the three species, contrary to the leveling off expected under resource limitation; (2) colonies of all sizes were equally likely to produce any number of alates, rather than having a size threshold for reproduction; (3) the scaling of soldier number was unrelated to alate production, but varied from isometry in N. braziliensis to negative allometry in L. labralis despite their similar defense strategies; (4) A. banksi had more immatures per worker and a higher maximum alate number per worker than the other species, suggesting that soldier loss may have allowed higher relative investment in colony growth and, possibly, reproduction. Termites can provide novel insights into life-history allocation strategies and their relation to social evolution, and should be better incorporated into sociobiological theory.

Reproduction and caste ratios under stress in trematode colonies with a division of labour

SUMMARYTrematodes form clonal colonies in their first intermediate host. Individuals are, depending on species, rediae or sporocysts (which asexually reproduce) and cercariae (which develop within rediae or sporocysts and infect the next host). Some species use a division of labour within colonies, with 2 distinct redial morphs: small rediae (non-reproducing) and large rediae (individuals which produce cercariae). The theory of optimal caste ratio predicts that the ratio of caste members (small to large rediae) responds to environmental variability. This was tested in Philophthalmus sp. colonies exposed to host starvation and competition with the trematode, Maritrema novaezealandensis. Philophthalmus sp. infected snails, with and without M. novaezealandensis, were subjected to food treatments. Reproductive output, number of rediae, and the ratio of small to large rediae were compared among treatments. Philophthalmus sp. colonies responded to host starvation and competition; reproductive output was higher in well-fed snails of both infection types compared with snails in lower food treatments and well-fed, single infected snails compared with well-fed double infected snails. Furthermore, the caste ratio in Philophthalmus sp. colonies was altered in response to competition. This is the first study showing caste ratio responses to environmental pressures in trematodes with a division of labour.