Daphnia magna microRNAs respond to nutritional stress and ageing but are not transgenerational

Abstract

Maternal effects, where the performance of offspring is determined by the condition of their mother, are widespread and may in some cases be adaptive. The crustacean Daphnia magna shows strong maternal effects: offspring size at birth and other proxies for fitness are altered when their mothers are older or when mothers have experienced dietary restriction. The mechanisms for this transgenerational transmission of maternal experience are unknown, but could include changes in epigenetic patterning. MicroRNAs (miRNAs) are regulators of gene expression that have been shown to play roles in intergenerational information transfer, and here, we test whether miRNAs are involved in D. magna maternal effects. We found that miRNAs were differentially expressed in mothers of different ages or nutritional state. We then examined miRNA expression in their eggs, their adult daughters and great granddaughters, which did not experience any treatments. The maternal (treatment) generation exhibited differential expression of miRNAs, as did their eggs, but this was reduced in adult daughters and lost by great granddaughters. Thus, miRNAs are a component of maternal provisioning, but do not appear to be the cause of transgenerational responses under these experimental conditions. MicroRNAs may act in tandem with egg provisioning (e.g., with carbohydrates or fats), and possibly other small RNAs or epigenetic modifications.

Genome-wide Methylation Patterns Under Caloric Restriction inDaphnia magna

“…For this, individuals were kept in artificial pond medium at 20°C and on a 12h:12h light:dark 298 cycle and fed 2.5 x 10 6 cells of the single-celled green algae Chlorella vulgaris daily. Following three generations of acclimatisation (detailed in [82]), 40 offspring from each mother were 300 isolated and split to form a replicate. Twenty were fed a normal diet of 5x10 6 algal cells/day and the remaining twenty that were fed a calorie restricted diet of 1x10 6 the reference was converted to clone 32 as for Hearn et al (2018) [82].…”

“…Following three generations of acclimatisation (detailed in [82]), 40 offspring from each mother were 300 isolated and split to form a replicate. Twenty were fed a normal diet of 5x10 6 algal cells/day and the remaining twenty that were fed a calorie restricted diet of 1x10 6 the reference was converted to clone 32 as for Hearn et al (2018) [82]. This was to increase mapping efficiency, and accuracy of the analysis, by reducing polymorphism 330 between the reference (assembled from a different clone) and our data.…”

Transcriptional response to kairomone exposure in Daphnia magna: A candidate gene approach

“…The identi cation of miRNAs in D. magna is relatively recent (Ünlü et al, 2015) and only two reports have aimed to identify changes in miRNA expression levels, one focusing on calorie restriction and ageing (Hearn et al, 2018) and a second looking across life stages (Coucheron et al, 2019). To determine a potential role for miRNAs in regulating gene expression in response to kairomone exposure, we took a targeted approach, monitoring the transcript levels of four miRNAs: miR-7, miR-34, miR-317, and miR-375.…”

“…Given the short timeframe required for these changes, epigenetic modi cations have been predicted to play a role as a regulatory mechanism (Harris et al 2012). With their well characterized life cycle, ease of laboratory-based cultivation, clonal reproductive capabilities, and the availability of whole genome sequence data and miRNA pro les (Colbourne et al 2011, Ünlü et al 2015, Hearn et al 2018, Coucheron et al 2019, Daphnia are ideal candidate organisms to probe for epigenetic regulatory mechanisms responsible for directing biological responses to changes in environmental cues.…”

“…In Daphnia, interest in identifying the biological roles for miRNAs have increased due to their connection to multiple stress-related cellular mechanisms. For example, Daphnia speci c stress responsive miRNAs associated with exposure to heavy metal (Chen et al 2016), changing nutritional conditions, (Hearn et al 2018) and ageing (Hu et al 2018) have recently been identi ed. To date, no miRNA studies linking abiotic stressors such as kairomone to changes in gene regulation have been reported.…”