Abstract
Migration is a widely observed phenomenon supported by morphological,
physiological and behavioural traits that vary with season. Moreover,
they also vary between sexes in a wide range of migrant species due to
different life-history requirements. Recently studies have unpacked the
genetic components underpinning migration in the marmalade hoverfly
(Diptera: Syrphidae) by detecting differential gene expression between
migrant and non-migrant females. Male hoverflies also migrate, but
changing sex ratios during their journeys in autumn, from approximately
50% females in northern Europe to ~90% the south,
suggest males are poorer long-distance migrants. To elucidate the
mechanisms underpinning this sex difference, we carried out
morphological, physiological, and transcriptomic characterisation of
actively migrating females and males caught as they traversed a high
mountain pass in the Pyrenees. Females and males show similar
physiological adaptations to migration such as hyperphagia and
resistance to starvation, but females display significantly higher
tolerance to cold and hypoxia, up-regulate a suite of immune genes, and
have lower wing loading values and a higher flight capacity. In
addition, age-related loss of flight muscle size indicates that females
are significantly older than male migrants, and transcriptomics indicate
that this increased longevity is centred on up-regulation of takeout and
the suppression of hormonal pathways maintaining reproductive diapause.
These results may be relevant to a swathe of insect migrants that show
sex biased migrations and influence population dynamics in many
beneficial and pest species world-wide.