Queens control caste allocation in the ant Cardiocondyla obscurior

Abstract

Social insect queens and workers can engage in conflict over reproductive allocation when they have different fitness optima. Here, we show that queens have control over queen-worker caste allocation in the ant Cardiocondyla obscurior , a species in which workers lack reproductive organs. We describe crystalline deposits that distinguish castes from the egg stage onwards, providing the first report of a discrete trait that can be used to identify ant caste throughout pre-imaginal development. The comparison of queen and worker-destined eggs and larvae revealed size and weight differences in late development, but no discernible differences in traits that may be used in social interactions, including hair morphology and cuticular odours. In line with a lack of caste-specific traits, adult workers treated developing queens and workers indiscriminately. Together with previous studies demonstrating queen control over sex allocation, these results show that queens control reproductive allocation in C. obscurior and suggest that the fitness interests of colony members are aligned to optimize resource allocation in this ant.

Sex- and caste-specific developmental responses to juvenile hormone in an ant with maternal caste determination

“…In contrast, all untreated, presumed worker-destined eggs developed into workers, indicating that worker-destined eggs are more easily identified than queen-destined eggs and/or more likely to end up in experiments because they outnumber queen-destined eggs in stock colonies. The accuracy of caste prediction in early developmental stages furthermore appears to be influenced by sampling precision, as a previous study showed higher prediction accuracy rates for queen-destined eggs, but lower rates for queen-destined first instar larvae (Schultner et al 2023). Caste prediction accuracy was lowest for methoprene-treated worker-destined eggs, but still relatively high at 80% (12/15), suggesting minor, if any, effects of methoprene on caste once it can be identified by caste-specific crystalline deposits.…”

“…Across all developmental stages and treatments, a higher proportion of queen-destined compared to worker-destined individuals survived until pupation (queen-destined: 44.5% (346/893), worker-destined: 38.1% (321/868); Fisher’s exact test, odds ratio=0.770, p=0.008); this was mostly driven by differential survival of castes in the third larval instar (Table 1). Adult workers do not discriminate between developing queens and workers (Schultner et al ., 2023), so differential treatment is unlikely to explain this difference. Instead, increased survival of queen-destined larvae may stem from size differences between the two castes (Oettler et al ., 2019; Schultner et al ., 2023).…”

“…Adult workers do not discriminate between developing queens and workers (Schultner et al ., 2023), so differential treatment is unlikely to explain this difference. Instead, increased survival of queen-destined larvae may stem from size differences between the two castes (Oettler et al ., 2019; Schultner et al ., 2023).…”

“…3% of ∼300 genera) in which complete worker sterility has evolved, meaning adult workers lack reproductive organs. We recently showed that in C. obscurior , queen- and worker-destined embryos and larvae can be distinguished in a non-invasive manner using caste-specific crystalline deposits (Schultner et al ., 2023). Queens appear to have full control over the production of daughter queens, which increases as queens age (Jaimes-Nino et al ., 2022), indicating that female caste is determined via maternal effects in this species (Schultner et al ., 2023).…”

“…We recently showed that in C. obscurior , queen- and worker-destined embryos and larvae can be distinguished in a non-invasive manner using caste-specific crystalline deposits (Schultner et al ., 2023). Queens appear to have full control over the production of daughter queens, which increases as queens age (Jaimes-Nino et al ., 2022), indicating that female caste is determined via maternal effects in this species (Schultner et al ., 2023). In addition to the two female castes, C. obscurior also produces two discrete male morphs – large, winged males and small, wingless males (Kinomura & Yamauchi, 1987; Heinze & Hölldobler, 1993; Oettler et al ., 2010).…”