Transcriptome analysis reveals nutrition‐ and age‐related patterns of gene expression in the fat body of pre‐overwintering bumble bee queens

Abstract

Many diapausing insects undergo a nutrient storage period prior to their entry into diapause. Bumble bee queens diapause as adults in the winter preceding their spring nest initiation period. Before diapause, they sequester glycogen and lipids, which they metabolize during the overwintering period. We used RNA sequencing to examine how age and nectar diet (specifically, the concentration of sucrose in nectar) impact gene expression in the pre‐overwintering bumble bee queen fat body, the “liver‐like” organ in insects with broad functions related to nutrient storage and metabolism. We found that diet on its own, and in combination with age, impacts the expression of genes involved in detoxification. Age was also a strong driver of gene expression, especially at earlier ages (up to 3 days). In addition to these molecular correlates of diet and age, we also found a putative molecular signature of diapause entry or preparation in adult queens in the oldest age group (12 days) fed the most sucrose‐rich diet, based on comparisons between our data set and another transcriptome data set from bumble bee queens. This transcriptomic pattern suggests that preparation for (or entry into) diapause might be in part mediated by nutritional state in bumble bee queens. Collectively, these findings show that there are molecular processes in the fat body that are responsive to sucrose levels in the diet and/or associated with age‐related maturational changes. A better understanding of these processes may shed light on important aspects of bumble bee biology, such as queen responses to nutritional and other forms of stress, and the factors that regulate their entrance into diapause.

Flexibility in the Critical Period of Nutrient Sequestration in Bumble Bee Queens

“…As we only examined weight and nutrient storage in our experiment, there might be additional sublethal consequences of starvation that we did not examine, which impact queen fitness. For example, limitation of nectar-derived sugar in the bumble bee queen diet during the pre-diapause period results in a downregulation of P450 enzyme expression, suggesting an impairment of immune function ( Costa et al 2020 ). Similar relationships between diet and immunity have been detected in other studies of bumble bees ( Roger et al 2017 ) and honey bees ( Alaux et al 2010 ; Corby-Harris et al 2019 ).…”

“…The ability to upregulate feeding might be particularly important during the period preceding diapause, given that failure to sequester adequate nutrients decreases the likelihood of surviving diapause ( Woodard et al 2019 ). There is also some evidence that queen nutritional status is positively associated with the onset of diapause, suggesting that nutrient sensing might contribute to preparation for, or entry into, diapause ( Costa et al 2020 ). Queens may also synthesize fatty acids from dietary sugar, and may convert fatty acids to glycogen via gluconeogenesis; these biochemical processes may allow queens to convert nutrients from nectar and pollen, respectively, to compensate for a lack of pollen or nectar in the diet.…”

“…However, in the wild, recovery would be dependent on the ability to find and access food resources, and might also be negatively impacted by the amount of energy use required to acquire those resources. Further, sublethal effects of inadequate nutrition are only beginning to be understood for bumble bees ( Moerman et al 2016 ; Dance et al 2017 ; Roger et al 2017 ; Leza et al 2018 ; Woodard et al 2019 ; Costa et al 2020 ), and might translate to reduced fitness for queens, particularly in landscapes where they are exposed to multiple stressors ( Dance et al 2017 ; Leza et al 2018 ; Costa et al 2020 ). Identifying the full breadth of ways that queen bumble bees respond to nutritional challenges, across all life stages, will help us better understand patterns of persistence and decline in this important pollinator group.…”

“…Our NS3d queens, for example, refer to their being nectar starved for 3 days; these queens received pollen for the duration of the experiment but were starved of artificial nectar for 3 days, then were switched to the control nectar diet for the remainder of the experiment. Bombus impatiens queen nutrient stores completely plateau by around age 12 days ( Alford 1969a ; Röseler and Röseler 1986 ; Woodard et al 2019 ; Treanore and Amsalem 2020 ), which might signify preparation for or the onset of diapause ( Costa et al 2020 ). We chose the timepoints of 3, 6, 9, and 12 days of age because they span the approximately 12-day pre-diapause, nutrient storage period.…”

“…There are also sublethal consequences of pre-diapause food availability. Sugar-poor diets during the pre-overwintering period cause a decrease in the expression of detoxification enzymes in the fat body, which suggests some impairment in this function under scenarios where floral nectar is limited ( Costa et al 2020 ). …”