Geographic patterns in colonial reproductive strategy in Myrmecina nipponica: Links between biogeography and a key polymorphism in ants

A Breeding System Derived From Asexual Queen Succession in Termite Colonies From Cold Climate Regions

In social insects, geographic variation is often accompanied not only by physiological changes but also by changes in their social system. In the subterranean termite Reticulitermes speratus that exhibits a sophisticated social system, colonies in subtropical and temperate areas are founded by a pair of primary king and queen derived from sexually produced alates. Some years after colony establishment, many neotenic queens are produced parthenogenetically, which is known as asexual queen succession (AQS). This strategy boosts reproduction without inbreeding. Here we show that subarctic populations of R. speratus, where colonies founded by alates cannot be sustained due to the cold conditions, undergo inbreeding rather than AQS, with colonies headed by numerous neotenic reproductives. Genetic analysis found that most neotenic queens were produced sexually in the subarctic populations, rather than asexually. Rearing experiments using colonies consisting only of nymphs (reproductive-destined individuals) and workers revealed that more nymphs successfully established as neotenic reproductives in the subarctic populations than in temperate populations, and that a higher number of individuals were maintained in the subarctic populations. These results suggest that sexually produced nymphs in subarctic populations are highly predisposed to develop into neotenic reproductives, whereas in temperate populations, their developmental potential is predominantly directed towards becoming alates. This study demonstrates that R. speratus has adjusted to colder climatic zones by changing its sophisticated AQS reproductive system into a secondary strategy to maintain colonies, elucidating the flexible adaptation and acclimation of reproductive systems in social insects.

Solitary foundation or colony fission in ants: an intraspecific study shows that worker presence and number increase colony foundation success

Dispersal and establishment strategies are highly variable. Each strategy is associated with specific costs and benefits, and understanding which factors favour or disfavour a strategy is a key issue in ecology and evolution. Ants exhibit several strategies of establishment, i.e. of colony foundation. Some species rely on winged queens that found new colonies alone when others found with accompanying workers (colony fission). The benefits conferred by these workers have been little studied and quantified, because comparing the costs and benefits of solitary foundation vs. colony fission is difficult when comparing different species. We investigated this using the ant Myrmecina graminicola, one of the few species that use both strategies. Young mated queens were allowed to found new colonies in the laboratory, with either zero (solitarily), two or four workers (colony fission). The presence of workers increased both survival and growth of the foundations over the first year, with more workers yielding higher growth. Few workers (as little as two workers) were sufficient to provide benefits, suggesting that in M. graminicola the strategy of colony fission may not dramatically decrease the number of new colonies produced compared to solitary foundation. Because queens performing solitary foundation or colony fission differ in dispersal (by flight vs. on foot), our results support the hypothesis that these two strategies of foundation coexist along a competition-colonization trade-off, where solitary foundation offers a colonization advantage, while colony fission has a competitive advantage.

Cooperation by ant queens during colony-founding perpetuates alternative forms of social organization

Abstract

Key social traits, like queen number in eusocial insect colonies, have long been considered plastic, but the recent finding that colony social organization is under strict genetic control in multiple ant lineages challenges this view. This begs the question of which hardwired behavioral mechanism(s) generate alternative forms of social organization during colony development. We addressed this question in the Alpine silver ant, Formica selysi, a species with two social forms determined by a supergene. Queens that carry exclusively the M haplotype are produced by and live in monogyne (= single-queen) colonies, whereas queens that carry at least one copy of the P haplotype are produced by and live in polygyne (= multiple-queen) colonies. With extensive field samplings and laboratory experiments, we show that both types of queens successfully establish colonies independently, without being accompanied by workers, but that they do so in contrasting ways. Monogyne queens were generally intolerant of other queens and founded colonies solitarily, whereas polygyne queens were mutually attracted to each other and mainly founded colonies cooperatively. These associations persisted for months after worker emergence, suggesting that cooperative colony-founding leads to permanent multiple queening. Overall, our study shows that queens of each social form found colonies independently in the field but that P-carrying queens are more likely to cooperate, thereby contributing to perpetuate alternative forms of social organization.

Significance statement

Understanding the genetic and behavioral underpinnings of social organization is a major goal in evolutionary biology. Recent studies have shown that colony social organization is controlled by supergenes in multiple ant lineages. But the behavioral processes linking the genotype of a queen to the type of colony she will form remain largely unknown. Here, we show that in Alpine silver ants, alternative supergene genotypes are associated with different levels of social attraction and tolerance in young queens. These hardwired differences in social traits make queens carrying the P supergene haplotype more prone to cooperate and form durable associations during independent colony-founding. These findings help explain how genetic variants induce alternative forms of social organization during the ontogeny of a colony. They also illustrate how simple phenotypic differences at the individual level can result in large differences at higher levels of organization.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00265-021-03105-1.