Queen Dominance May Reduce Worker Mushroom Body Size in a Social Bee

Abstract

The mushroom body (MB) is an area of the insect brain involved in learning, memory, and sensory integration. Here, we used the sweat bee Megalopta genalis (Halictidae) to test for differences between queens and workers in the volume of the MB calyces. We used confocal microscopy to measure the volume of the whole brain, MB calyces, optic lobes, and antennal lobes of queens and workers. Queens had larger brains, larger MB calyces, and a larger MB calyces:whole brain ratio than workers, suggesting an effect of social dominance in brain development. This could result from social interactions leading to smaller worker MBs, or larger queen MBs. It could also result from other factors, such as differences in age or sensory experience. To test these explanations, we next compared queens and workers to other groups. We compared newly emerged bees, bees reared in isolation for 10 days, bees initiating new observation nests, and bees initiating new natural nests collected from the field to queens and workers. Queens did not differ from these other groups. We suggest that the effects of queen dominance over workers, rather than differences in age, experience, or reproductive status, are responsible for the queen-worker differences we observed. Worker MB development may be affected by queen aggression directly and/or manipulation of larval nutrition, which is provisioned by the queen. We found no consistent differences in the size of antennal lobes or optic lobes associated with differences in age, experience, reproductive status, or social caste.

The Loss of Sociality Is Accompanied by Reduced Neural Investment in Mushroom Body Volume in the Sweat Bee Augochlora Pura (Hymenoptera: Halictidae)

“…These brain differences do not result from differences in social experience, because all individuals were solitary at the time they were collected, which suggests that the MB calyx size differences reflect species-specific patterns of neural investment. This is consistent with previous studies of socially polymorphic bees (those that can nest socially and solitarily) which also found smaller relative MB calyx size in solitary reproductives relative to social queens, but these did not control for potential effects of social interactions on MB size ( Smith et al 2010 , Rehan et al 2015 ; but see Jaumann et al 2019 ).…”

“…To test whether brain size relative to body size differed between species, we standardized individual whole brain volume to individual body size by calculating a correction factor that was applied to each bee: mean body size of all bees in the study divided by the individual’s body size. This correction factor was then multiplied to brain volume for each bee to calculate size-corrected whole brain volume ( Jaumann et al 2019 ). We used an independent samples t -test with equal variances not assumed to compare the two species; all variables fit a normal distribution with Shapiro–Wilk test P -values >0.05.…”

“…However, recent intraspecific comparisons in social insects suggest that while the evolutionary origins of large MBs may predate sociality, differences in social behavior can still lead to differences in MB investment. For example, differences in MB volume associated with dominance status within social species demonstrate a social influence on MB size ( Molina et al 2007 , 2008 ; O’Donnell et al 2008, 2017 ; Smith et al 2010 ; Rehan et al 2015 ; Jaumann et al 2019 ; Pahlke et al 2019 ). A direct test of whether social behavior leads to increased MB investment relative to solitary living should compare closely related social and solitary species.…”

Differential investment in visual and olfactory brain regions mirrors the sensory needs of a paper wasp social parasite and its host

“…comm.). Our results of high investment in olfaction and sensory processing are consistent with other studies that show preferential brain investment in brain region associated needed for effective communication, maintenance of division of labor, learning and memory (Farris et al, 2001;Gronenberg et al, 1996;Jaumann et al, 2019;O'Donnell et al, 2015;Rehan et al, 2015;Seid et al, 2005;Smith et al, 2010). Host queens had significantly higher antennal lobes, as they spend most of their time on the nest and communicate with incoming subordinate workers, who spend more time foraging off the nest (Molina & O'Donnell, 2008).…”

Age-related mushroom body expansion in male sweat bees and bumble bees

“…These changes are similar to those observed in our mature B. impatiens males. However, experience-expectant neuroplasticity is absent in M. genalis females 54 (though see 55 ), which also lack age-related task specialization 25 , 56 . Our finding that mushroom body expansion occurs with maturation in male M. genalis suggests that experience-expectant neuroplasticity can occur in males, even when it is absent in females.…”