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.成熟过程是一类关键的发育进程，其发生的年龄与体型大小对生物适合度具有重要影响。尽管成熟过程存在显著的变异性，但学界已提出，包括最小体型或状态阈值在内的特定机制，是广泛生物类群中该过程的潜在调控基础。近期研究证据表明，阈值本身可能具有发育可塑性；而在水蚤（Daphnia pulex）中，成熟过程究竟遵循阈值模式，还是属于更贴近速率调控的渐进过程，目前尚无定论。本研究通过cDNA微阵列（cDNA microarray）实验，对成熟前及成熟过程中四个龄期的基因表达变化开展了比较分析。为明确基因表达变化呈渐进式还是离散式，研究将发育阶段作为统计变量，分别以不连续和连续两种方式进行处理。连续变量分析鉴定出的显著差异表达基因数量（45个）多于不连续变量分析（11个）。多数基因（包括编码组蛋白、转录相关因子与细胞周期调控因子的基因，以及一个推定的发育激素基因）的表达量从第一龄至第四龄呈持续上升或下降趋势，这提示成熟过程是一个持续且渐进的过程。有三个编码卵黄蛋白原/超氧化物歧化酶融合蛋白的基因，其表达量在第二龄后出现上升，且与假定的成熟阈值出现时间相吻合，但即便这些基因的表达变化仍呈连续模式。