◆◆◆◆◆◆◆◆◆◆◆◆◆◆CHAPTER 25 ◆◆◆◆◆◆◆◆◆◆◆◆◆ Contributions of Morphometrics to Medical Entomology J.-P. Dujardin1 and D.E. Slice2 1Génétique et Evolution des Maladies Infectieuses, UMR CNRS/IRD, Montpellier, France 2 Institute for Anthropology, University of Vienna,Vienna,Austria “J’ai besoin de savoir que tout n’est pas confondu. (I need to know that everything is not confounded)” —Jean Tardieu, in La Part de l’Ombre 25.1 INTRODUCTION 25.1.1 From Dimensions to Biology In the absence of artifactual variation, a distance between two Morphometrics quantitatively describes the morphological anatomical landmarks or their relative position to other such variation of objects.When applied to biological forms, it is points (see Figs. 25.1 and 25.2), depend on the morphological a particular field of biometrics. In medical entomology, development of the organism under study; their variation with where a major interest is the biology of insects in their geography is arguably an effect of both environmental influ- natural environment, morphometrics might be considered ence and adaptive changes; and their changes from one species as a tool for quantifying the phenotypic variation of an to another reflect the process of natural evolution.When prop- organism. Morphometrics focuses on variation, its parame- erly analyzed, metric-trait variation allows one to read some terization, and relation to extrinsic factors. As long as biological and evolutionary information embedded in the phenotypic variation has environmental and/or genetic morphology [79,81,91]. One of the earliest morphometric causes, morphometrics can help detect local adaptations and studies is illustrative. After a severe storm in February 1898, genetic divergence among populations. Morphometric among the moribund sparrows taken to the laboratory by characters are related to growth and development, and they Bumpus, some survived, others died. Examining a few meas- are usually continuous.Traditionally, they were estimates of urements of their skeleton, Bumpus showed that “the birds distances between anatomical points called landmarks. More which perished, perished not through accident, but because recently, they have come to be the coordinates of these they were physically disqualified,’’ and “the birds which sur- landmarks in a given system of orthogonal axes. We will vived, survived because they possessed certain physical charac- present here some concepts and statistical analyses related to ters’’ [64].Thus, a simple set of measurements was able to illus- the use of these data, insisting on their biological relevance, trate the Darwinian concept of selection for the most fit. with some examples of applications in medical entomology. Morphometrics has this ability to make visible to us many Both traditional and geometric approaches will be present- aspects of the biology of an organism, such as its physiology,its ed. Special attention is given to applications involving pathology, and its phenotypic or genetic evolution. Triatominae (Hemiptera: Reduviidae), the vectors of Chagas disease in Latin America (see chapter in this book) 25.1.2 Tradition and Modernity and Phlebotominae, the vectors of leishmaniasis. Finally, The virtues of traditional morphometrics are today improved some information will be given about morphometric by the introduction of geometric techniques [83]. software. Morphometrics is often presented as “traditional,’’ making Encyclopedia of Infectious Diseases: Modern Methodologies, by M.Tibayrenc Copyright © 2007 John Wiley & Sons, Inc. 435 436 ◆ ENCYCLOPEDIA OF INFECTIOUS DISEASES: MODERN METHODOLOGIES individuals or populations. Shape can also be studied by other direct techniques exploring outlines, textures, surface pat- terning, or even internal configuration of a form:These tech- niques will not be discussed in this chapter. In spite of these advantages and the attractiveness of modern morphometrics, it has yet to gain popularity in medical entomology, where traditions may be hard to move. 25.2 CAUSES OF METRIC VARIATION? Fig. 25.1. Distances between anatomical landmarks (left) are advan- Morphometric variation is under the influence of physio- tageously replaced by coordinates of these landmarks in a given sys- logical (or pathological) status, adaptive changes, and genet- tem of orthogonal axes (right). ic differences. Whereas different molecular markers applied to an insect will be differently affected by the environ- ment—and some could be completely neutral—the metric use of limited sets of measurements, or “modern’’ (or “geo- characters are generally supposed to be of both environ- metric’’), making use of total geometric information (see mental and genetic origin.The drawback is that there is no Figs. 25.1 and 25.2). This difference, which derives mainly magic science to make the correct partition between these from the kind of metric data (distances versus coordinates), ultimate causes of metric variation:The genetic make-up of has generated a “revolution’’ [3,83]. Improvements or novel- a population or/and its environment.The cause of morpho- ties exist indeed at various levels, the most important one metric variation cannot be found in the metric variation being the direct description of shape itself.After some math- itself, it has to be searched by other methods and may ematical processing, the geometric figures represented by the become the object of an inquiry, itself. It is, however, possi- landmarks are compared as different point sets between ble to obtain from the data some helpful insight. As a first step to remove heterogeneous environmental influences and focus more on genetic differences, one could rear a complete generation of various samples under the same laboratory conditions [16,19]. It is however important to take into account possible genetic drift effects (number of founders) or even microenvironmental influences within laboratory conditions.A more speculative approach to tentatively parti- tion causes of metric variation is the separate analysis of size and shape, based on the idea that shape would have less envi- ronmental variance [32]. If the main interest is to focus on environmentally induced changes, the study of fluctuating asymmetry of bilateral characters is an elegant, but challeng- ing, approach (see Fig. 25.12). Adapted methodology (the study of bilateral structures) and accurate statistical tech- niques exist to reveal these environmentally induced changes [74], which are now applied to geometric morphometrics [55,66,89]. Fig. 25.2. Screenshot of a landmark collection session under the 25.2.1 Physiological Causes COO program (http://www.mpl.ird.fr/morphometrics). Top left The main cause of metric differences related to physiology is window figures a small database gathering relevant informations. obviously differential growth, when this growth heterogene- Yellow dots on the insect are the landmarks of the wing, labeled in ity is of environmental origin. Depending on more or less the order of collection. The bug is an undissected, dry pinned favorable environmental conditions, and on aging in verte- Rhodnius prolixus. It is a South and Central American species of the brates, individuals may be more or less developed. For con- subfamily Triatominae (Hemiptera, Reduviidae). After Tr iatoma specific individuals, traditional morphometrics proposes a set infestans (see Fig. 25.12), it is the main vector of Trypanosoma cruzi, the causative agent of Chagas disease.The collection of landmarks is of statistical methods to remove this effect of age or growth performed on a digital picture, using the “mouse-clic’’ with a dedi- from their metric variation. Scaling for size is interesting cated program (here COO,see http://www.mpl.ird.fr/morphomet- when one wants to remove the effects of physiological dif- rics; a more versatile and frequently used program is TPSdig, see ferences and concentrate on other causes of intraspecific vari- http://life.bio.sunysb.edu/morph/). ation. In that case, the size estimator—the one that’s effect CHAPTER 25 CONTRIBUTIONS OF MORPHOMETRICS TO MEDICAL ENTOMOLOGY ◆ 437 will be removed from the metric variation—should be con- regarded as an analytical process increasing our capacity to structed from the dimensions of the anatomical structure interpret metric variation. under study, not from an external indicator (weight, etc.).An external indicator of size is acceptable when the objective is 25.2.4 Genetic Causes to study the meaning of size variation itself. A complete Interspecific metric differences most probably have a genetic review of these methods for traditional morphometrics may origin.The nature of these genetic differences is not within be found in Ref. [54]. the scope of this introduction, but their effects on metric traits deserve some discussion here.Although there is no spe- 25.2.2 Pathological Causes cial metric feature marking the difference between species, Some mutation or toxin may affect the morphogenesis of this topic is influenced by common ideas found in the liter- some individuals. Morphometrics is not always required to ature, not completely true and not completely false. detect such changes, as they generally produce obvious, vis- ible deformations. Many times, pathological causes produce 25.2.4.1 The amount of differences The level of inter- extreme individuals (“outliers’’).They may be removed from specific differences is generally (much) higher than the cor- the dataset or included provided more robust statistical tech- responding intraspecific variation, even across geographic niques