Mabee P M, Olmstead K L, Cubbage C C
Department of Biology, University of South Dakota, Vermillion 57069, USA.
Evolution. 2000 Dec;54(6):2091-106. doi: 10.1111/j.0014-3820.2000.tb01252.x.
Heterochrony is widely regarded as an important evolutionary mechanism, one that may underlie most, if not all, morphological evolution, yet relatively few studies have examined variation in the sequence of development. Even fewer studies have been designed so that intraspecific variation in the relative sequence of developmental events can be assessed, although this variation must be the basis for evolutionary change. Intraspecific variation in developmental ossification sequences was documented from the zebrafish (Danio rerio) by Cubbage and Mabee (1996) and from the Siamese fighting fish (Betta splendens) by Mabee and Trendler (1996), but a quantitative analysis of the patterns within this variation was not made. Here, we quantify the effect of rearing temperature on the sequence of ossification and characterize the levels and patterns of intraspecific variation in these fishes. For Danio, there were no temperature effects on the sequence of bone development across the cranium, cranial region development, cartilage versus dermal bones, or lateral line bone versus nonassociated bones. Likewise the level of variation in relative sequence (position) of ossification was low, about two ranks, across temperatures. At higher temperatures, we found higher levels of variation in iterated cranial bones and less in bones forming early in the sequence. No temperature effects on variation were found among regions, between lateral line-associated bones and nonassociated bones, between median and paired bones, or across the entire sequence, indicating concordant variability among the three temperatures. Individual bones with the highest levels of variability were not consistent among temperatures. Baseline patterns of intraspecific variation in Danio were compared to those of Betta. For both species, the level of intraspecific variation in sequence position was low and the variability of cranial bones was concordant. Individual bones with the highest levels of variability were not consistent between species. In both species, variation was widespread (distributed evenly across the sequence). We used comparisons (among regions, between dermal and cartilage bones, between lateral line-associated and other bones, between median and paired bones, between iterated and noniterated bones, between feeding-associated bones and others) to see which subsets were most variable and thus potentially useful in predicting high levels of evolutionary change. The only subset of bones that was significantly more variable than others was cartilage bones. If interspecific patterns are parallel to these intraspecific differences, cartilage bones would be expected to show higher levels of heterochrony. Although concordance across the cranial ossification sequence and among regions in Danio, Betta, and two other teleosts, Oryzias and Barbus, suggests an evolutionarily conserved pattern of ossification, identity in sequence position across taxa was not observed for any bone. Thus, variation existed in sequence position across temperatures and species. Intraspecific variation of this sort may influence the morphological outcome and evolutionary trajectories of species.
异时性被广泛认为是一种重要的进化机制，它可能是大多数（即便不是全部）形态进化的基础，然而相对较少的研究考察了发育序列的变化。设计来评估发育事件相对序列种内变异的研究更少，尽管这种变异必定是进化变化的基础。Cubbage和Mabee（1996年）记录了斑马鱼（Danio rerio）发育骨化序列的种内变异，Mabee和Trendler（1996年）记录了暹罗斗鱼（Betta splendens）的此类变异，但未对这种变异中的模式进行定量分析。在此，我们量化饲养温度对骨化序列的影响，并描述这些鱼类种内变异的水平和模式。对于斑马鱼而言，温度对整个颅骨的骨发育序列、颅骨区域发育、软骨与膜骨，或侧线骨与非相关骨均无影响。同样，在不同温度下，骨化相对序列（位置）的变异水平较低，约为两个等级。在较高温度下，我们发现重复的颅骨中变异水平较高，而在序列中早期形成的骨中变异水平较低。在不同区域之间、侧线相关骨与非相关骨之间、正中骨与成对骨之间，或整个序列中，均未发现温度对变异有影响，这表明在这三个温度下变异具有一致性。不同温度下变异水平最高的单个骨并不一致。将斑马鱼种内变异的基线模式与斗鱼的进行了比较。对于这两个物种，序列位置的种内变异水平都较低，且颅骨的变异性具有一致性。不同物种间变异水平最高的单个骨并不一致。在这两个物种中，变异分布广泛（在整个序列中均匀分布）。我们通过比较（在不同区域之间、膜骨与软骨骨之间、侧线相关骨与其他骨之间、正中骨与成对骨之间、重复骨与非重复骨之间、与摄食相关的骨与其他骨之间）来观察哪些子集的变异性最大，从而可能有助于预测高水平的进化变化。唯一变异性显著高于其他子集的骨是软骨骨。如果种间模式与这些种内差异平行，那么软骨骨预计会表现出更高水平的异时性。尽管斑马鱼、斗鱼以及另外两种硬骨鱼（稻田鱼和魮属鱼）的颅骨骨化序列以及不同区域之间具有一致性，表明存在一种进化上保守的骨化模式，但在任何骨上均未观察到不同分类单元在序列位置上的一致性。因此，在不同温度和物种之间，序列位置存在变异。这种种内变异可能会影响物种的形态结果和进化轨迹。
