Comparative analysis of larval growth in Lepidoptera reveals instar-level constraints

1. Juvenile growth trajectories evolve via the interplay of selective pressures on age and size at maturity, and developmental constraints. In insects, the moulting cycle is a major constraint on larval growth trajectories.
2. Surface area to volume ratio of a larva decreases during growth, so renewal of certain surfaces by moulting is likely needed for the maintenance of physiological efficiency. A null hypothesis of isometry, implied by Dyar’s Rule, would mean that the relative measures of growth remain constant across moults and instars.
3. We studied ontogenetic changes and allometry in instar-specific characteristics of larval growth in 30 lepidopteran species in a phylogenetic comparative framework.
4. Relative instar-specific mass increments (RMI) typically, but not invariably, decreased across instars. Ontogenetic change in RMIs varied among families with little within-family variation. End-of-instar growth deceleration (GD) became stronger with increasing body size across instars. Across-instar change in GD was conserved across taxa. Ontogenetic allometry was generally non-isometric both in RMI and GD.
5. Results indicate that detailed studies on multiple species are needed for generalizations concerning growth trajectory evolution. Developmental and physiological mechanisms affecting growth trajectory evolution show different degrees of evolutionary conservatism, which must be incorporated into models of age and size at maturation.
Methods
Phenotypic data were collected by measuring growing larvae under controlled laboratory conditions. Data have been converted in a format that can be used in statistical analyses.

1. 幼体生长轨迹（juvenile growth trajectories）的演化，由成熟时年龄与体型所受选择压力（selective pressures）与发育限制（developmental constraints）之间的相互作用驱动。在昆虫类群中，蜕皮周期（moulting cycle）是制约幼虫生长轨迹的主要因素。
2. 幼虫在生长过程中，其表面积体积比（surface area to volume ratio）会逐渐降低，因此通过蜕皮实现特定体表的更新，大概率是维持生理效率的必要条件。由戴氏法则（Dyar’s Rule）推导得出的等速生长原假设，意味着在各蜕皮周期与龄期内，生长的相对度量保持恒定。
3. 本研究在系统发育比较框架（phylogenetic comparative framework）下，针对30种鳞翅目物种（lepidopteran species）幼虫生长的龄期特异性特征，分析了其个体发育变化与异速生长（allometry）规律。
4. 龄期特异性相对体重增量（RMI，relative instar-specific mass increments）通常（但并非绝对）会随龄期增加而降低。RMIs的个体发育变化在各科之间存在差异，但科内变异极小。龄末期生长减速（GD，end-of-instar growth deceleration）程度会随各龄期的体型增大而加剧，且该跨龄期的GD变化在各分类群间具有进化保守性。整体而言，RMI与GD的个体发育异速生长均普遍不符合等速生长模式。
5. 研究结果表明，若要得出关于生长轨迹演化的普适性结论，需针对多个物种开展精细化研究。影响生长轨迹演化的发育与生理机制，其进化保守性程度存在差异，这一特性必须被纳入成熟时年龄与体型的相关模型中。
方法：本研究通过在受控实验室条件下测量生长中的幼虫，收集表型数据（phenotypic data）。相关数据已转换为可用于统计分析（statistical analyses）的格式。