Variance in developmental event timing is greatest at low biological levels: implications for heterochrony

Intraspecific variation in developmental event timing is common and may be the raw material from which heterochronies (altered timing of developmental events between ancestors and descendants) arise. However, our understanding of how variance in intraspecific developmental event timing is distributed across different hierarchical, biological levels is poor, despite recent evidence suggesting a genetic basis for such inter-individual differences. In the present study, we used high (temporal and spatial) resolution bio-imaging of the entire embryonic development of the pond snail, Radix balthica, to investigate how variance was partitioned between the biological levels of interpopulation, inter-egg mass and individual, with respect to the relative timing of four key developmental events (four-cell division, a discrete heart beat, capsule rupture, and hatching), as well as egg volume (an index of maternal investment), hatchling size and shape (to compare embryonic with post-hatch variance). We found that the timing of all but one (four cell division) of the developmental events, together with all measures of hatchling size and shape, had most variance partitioned at the individual level; variance at the interpopulation level was surprisingly low despite sampling populations from across the geographical range of R.balthica. These patterns highlight the importance of studying the distribution of variance in developmental event timing with the aim of understanding how it might drive organismal ecology and evolution.(c) 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 581-590.