Biomedical subjects

D J Emlen

Publications and source records attributed to D J Emlen.

5 recordsLinked to original sources

PubMed2006

Insulin signaling and limb-patterning: candidate pathways for the origin and evolutionary diversification of beetle 'horns'.

Beetle 'horns' are rigid outgrowths of the insect cuticle used as weapons in contests for access to mates. Relative to their body size, beetle horns can be enormous. They protrude from any of five different regions of the head or thorax; they are curved, straight, branched or bladed; and their development is often coupled with the nutrient environment (male dimorphism) or with sex (sexual dimorphism). Here, we show that this extraordinary diversity of horns can be distilled down to four trajectories of morphological change--horn location, shape, allometry and dimorphism--and we illustrate how the developmental mechanisms regulating horn growth could generate each of these types of horn evolution. Specifically, we review two developmental pathways known to regulate growth of morphological structures in Drosophila and other insects: a limb-patterning pathway that specifies the location and shape of a structure, and the insulin pathway, which modulates trait growth in response to larval nutrition. We summarize preliminary evidence indicating that these pathways are associated with the development of beetle horns, and we show how subtle changes in the relative activities of these two pathways would be sufficient to generate most of the extant diversity of horn forms. Our objective is to intuitively connect genotype with phenotype, and to advocate an informed 'candidate gene' approach to studies of the developmental basis of evolution. We end by using this insight from development to offer a solution to the long-standing mystery of the scarabs: the observation by Darwin, Lameere, Arrow and others that this one family of beetles appeared to have a 'special tendency' towards the evolution of horns.

Animals↗

PubMed2001

Costs and the diversification of exaggerated animal structures.

Sexual selection can favor production of extravagant ornaments and weapons in the contest for access to the opposite sex. Existing explanations for the diversity of sexually selected structures focus on reproductive benefits conferred by particular ornament or weapon morphologies. Here, I show that costs of weapon production also may drive patterns of weapon evolution. In beetles, production of horns reduces the size of neighboring morphological structures (antennae, eyes, or wings, depending on the location of the horns), and these tradeoffs reveal unexpected functional associations between ecology and horn morphology. This study illustrates a critical but overlooked role of costs in sexual selection and has implications for understanding the evolution of animal morphology.

Animals↗

PubMed2000

The development and evolution of exaggerated morphologies in insects.

We discuss a framework for studying the evolution of morphology in insects, based on the concepts of "phenotypic plasticity" and "reaction norms." We illustrate this approach with the evolution of some of the most extreme morphologies in insects: exaggerated, sexually selected male ornaments and weapons, and elaborate social insect soldier castes. Most of these traits scale with body size, and these scaling relationships are often nonlinear. We argue that scaling relationships are best viewed as reaction norms, and that the evolution of exaggerated morphological traits results from genetic changes in the slope and/or shape of these scaling relationships. After reviewing literature on sexually selected and caste-specific structures, we suggest two possible routes to the evolution of exaggerated trait dimensions: (a) the evolution of steeper scaling relationship slopes and (b) the evolution of sigmoid or discontinuous scaling relationship shapes. We discuss evolutionary implications of these two routes to exaggeration and suggest why so many of the most exaggerated insect structures scale nonlinearly with body size. Finally, we review literature on insect development to provide a comprehensive picture of how scaling relationships arise and to suggest how they may be modified through evolution.

Animals↗

PubMed1999

The developmental basis for allometry in insects.

Within all species of animals, the size of each organ bears a specific relationship to overall body size. These patterns of organ size relative to total body size are called static allometry and have enchanted biologists for centuries, yet the mechanisms generating these patterns have attracted little experimental study. We review recent and older work on holometabolous insect development that sheds light on these mechanisms. In insects, static allometry can be divided into at least two processes: (1) the autonomous specification of organ identity, perhaps including the approximate size of the organ, and (2) the determination of the final size of organs based on total body size. We present three models to explain the second process: (1) all organs autonomously absorb nutrients and grow at organ-specific rates, (2) a centralized system measures a close correlate of total body size and distributes this information to all organs, and (3) autonomous organ growth is combined with feedback between growing organs to modulate final sizes. We provide evidence supporting models 2 and 3 and also suggest that hormones are the messengers of size information. Advances in our understanding of the mechanisms of allometry will come through the integrated study of whole tissues using techniques from development, genetics, endocrinology and population biology.

Animals↗

PubMed1998

Competition among body parts in the development and evolution of insect morphology.

Changes in form during ontogeny and evolution depend in large measure on changes in the relative growth of the various parts of the body. The current consensus in developmental biology is that the final size of appendages and internal organs is regulated autonomously, within the structure itself. Size regulation of body parts typically requires no external control and is thought to be relatively insensitive to signals from the developmental environment. We show in two very different systems, butterfly wings and beetle horns, that experimentally induced changes in the allocation of developmental resources to one trait produces compensatory changes in the relative sizes of other traits. These findings illustrate that interaction among body parts in development is part of the mechanism of size regulation of those parts. Furthermore, in the case of beetle horns, we show that the tradeoff in size is manifest as a significant negative genetic correlation among the involved body parts and, therefore, constitutes a developmental source of genetic constraint on the evolution of body form.

Animals↗