Data from: Transcriptional changes during Daphnia pulex development indicate that the maturation decision resembles a rate more than a threshold

Maturation is a critical developmental process, and the age and size at
which it occurs have important fitness consequences. Although maturation
is remarkably variable, certain mechanisms, including a minimum size or
state threshold, are proposed to underlie the process across a broad
diversity of taxa. Recent evidence suggests that thresholds may themselves
be developmentally plastic, and in the crustacean Daphnia pulex it is
unclear whether maturation follows a threshold or is a gradual process
more akin to a rate. Changes in gene expression across four instars before
and during maturation were compared in a cDNA microarray experiment.
Developmental stage was treated statistically both as a discontinuous and
a continuous variable, to determine whether genes showed gradual or
discrete changes in expression. The continuous analysis identified a
greater number of genes with significant differential expression (45) than
the discontinuous analysis (11). The majority of genes, including those
coding for histones, factors relating to transcription and cell cycle
processes, and a putative developmental hormone showed continuous
increases or decreases in expression from the first to the fourth instar,
suggestive of a prolonged and gradual maturation process. Three genes
coding for a fused Vitellogenin/super-oxide dismutase showed increases in
expression following the second instar and coincided with the posited
maturation threshold, but even their expression increased in a continuous
fashion.