The evolution of female wing pigmentation in dragon- and damselflies (Insecta: Odonata)

Ana Laura Martínez-García, Alex Córdoba-Aguilar y Martín A. Serrano-Meneses

In 1871, Charles Darwin used the theory of sexual selection to explain why males of several animal taxa exhibited extravagant traits. Therefore traits like horns, conspicuous colours, long tails or appendages, protuberances or elongated mandibles were labeled as sexual traits (ST).

A notable ST exhibited by adult males of several dragon- and damselfly taxa (odonates) is wing pigmentation. Although the sexual function of such trait is relatively well documented for several taxa (albeit most studies have concentrated on damselflies, particularly in calopterygids), very little is known about its origins. However, using odonates as a study group, M. A. Serrano-Meneses has recently shown in a phylogenetic comparative study, that male wing pigmentation in male odonates is more likely to evolve if high levels of sexual selection evolve first.

Wing pigmentation, nonetheless, is not exclusively exhibited by males; females of several odonate taxa also exhibit wing pigmentation. The images, for instance, show taxa in which both males and females have pigmented wings (females on the left, males on the right in every image). Although it is known that the trait can signal female reproductive potential in at least one species (in Calopteryx haemorrhoidalis wing pigmentation is negatively correlated with parasite burden), it is not known why wing pigmentation evolves in females.

In this study, Ana Laura Martínez investigated the origin and direction of the evolution of female wing pigmentation in odonates. The main objective was to test whether male and female wing pigmentation evolved in a correlated manner. To this end, Ana Laura Martínez used the photographs of 146 odonate taxa in order to determine whether males and/or females exhibited wing pigmentation (absence: 0; presence: 1). She also used a recent phylogenetic tree, along with a maximum-likelihood method, to first reconstruct the ancestral stages of both males and females. The results of such set of analyses suggest that the ancestral stages did not exhibit wing pigmentation. In a second step, Ana Laura Martínez used a directional phylogenetic comparative method (Bayes Traits - Discrete) in order to test whether male and female wing pigmentation co-evolved.

The results show that male and female wing pigmentation evolved in a correlated manner: females acquired wing pigmentation after males had acquired the trait (due to increasing levels of sexual selection). Nonetheless, once female wing pigmentation is acquired, it may be lost over time...only to be acquired again. Why does this happen? Perhaps the costs associated to maintaining the trait are too high for them. There may be selection against the acquisition of the trait as strong as selection favouring its acquisition via genetic correlation.

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Distribution and endemicity patterns of Mexican dragon- and damselflies (Insecta: Odonata).

Distribution patters are important because they illustrate the history, in terms of time and space, of living organisms. The importance of the interaction between life, time and space, has been summarised by Léon Croizat: “Earth and life evolve together”. Much can be gained by investigating the distribution patterns of living organisms in nature, because one may find the explanations to the constant repetition of their distributions.

Biogeography studies the distribution (in time and space) of living organisms. It also investigates the causes and processes that determine such patterns. In 1820, de Candolle proposed the system of biogeographic regions. He based his observations on plants, the composition of regions and their relationships to climate. He also recognised a number of taxa that exclusively occupy a region; these taxa are known as ‘endemic taxa’. In other words, endemic taxa are restricted to a region, due to reproductive- or geographic isolation, or both.

Ecological niche models are robust tools used to predict the potential distribution of a given species, or that of a group of taxa. In order to estimate a potential distribution, these models use the presence records of a given species, as well as data on the location and date of observation of each individual.

Biogeography models have been used to plan conservation strategies, since they can be used to identify biodiversity hotspots or areas in need of protection. This is why it is important to identify the patterns of distribution of endemic taxa.

Freshwater is one of the most diverse ecosystems on the planet. One of the main reasons for the high levels of diversity of freshwater ecosystems is the way in which these are arranged: as ‘islands’. In such arrangement, islands are separated both by distance and altitude, which promotes diversification and speciation. Nowadays these ecosystems are severely threatened by human activities and climate change (among other reasons).

Dragon- and damselflies are insects of the order Odonata. These insects depend on freshwater ecosystems in order to reproduce and to develop from larvae to adults. Once the adult emerges, it leaves the water, develops two pairs of wings and tries to mate: males usually patrol the ponds or streams in search for females, or defend areas to which females are attracted, while the latter will come near the water in search for males. Once the females mate, they will fertilise their eggs before dropping them in the water or inserting them into a plant’s tissue.

Lucía del Carmen Salas Arcos is a first year MSc student at the Centro Tlaxcala de Biología de la Conducta (Universidad Autónoma de Tlaxcala, Mexico). She is supervised by Martín Alejandro Serrano-Meneses and by Enrique González Soriano (Universidad Nacional Autónoma de México, Mexico). Lucía’s work aims to determine the distribution patterns of endemic dragon- and damselfly taxa of Mexico, as well as their potential distribution (using ecological niche models).

The current information on the distribution of endemic Odonata of Mexico is mostly circumstantial. Therefore Lucía will gather data and produce results that will highlight not only the distribution of fragile taxa, but also those areas in need of desperate conservation efforts. To this end, Lucía will (i) elaborate an updated listing of endemic Odonata of Mexico, (ii) elaborate distribution maps of these taxa (using Diva GIS 7.2.3) by superimposing vegetation, weather and temperature layers, and (iii) model, along with Miguel Rivas Soto, the potential distribution of taxa, as estimated under the ecological niche criteria. Taken together, the information produced by Lucía’s study will be used to implement future, more focused and efficient conservation plans.

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VARIATION AND TRAITS RELATED TO THE MATING SYSTEMS OF THE SNOWY PLOVER (Charadrius nivosus)

We tend to think that sexual reproduction is a harmonious process in which males and females cooperate in order to increase the number of their descendants. Nevertheless, this is not always the case. During the reproductive process both males and females may have different reproductive interests; for instance, in some cases males may desert their offspring in order to mate again. This behaviour can be seen as disadvantageous for these males, since it may reduce the probability of survival of its offspring. However, these “disadvantageous” behaviours may be passed on from parents to offspring. The disparity in the reproductive interests between the sexes was formally termed “sexual conflict” by Geoff Parker in 1979. 

Deserting the offspring may be advantageous for the deserting parent, but it usually is disadvantageous for the parent left providing all the care. The deserting parent will have more time to find a new partner, which will increase his/her reproductive success. The parent left providing all the care will have to spend a considerable amount of energy, as he/she will have to provide the care of both parents and will not have time to re-mate until the offspring are mature enough. In the long term, both males and females will not agree on the number of copulations or partners. This, in turn, will shape the mating system (or systems) exhibited by a given species. 

The studies that have investigated the selective forces that shape a given mating system lay on the grounds of evolutionary- biology and ecology. Several theories have been proposed to explain the occurrence of a given mating system, and these are still at the core of the interests of modern evolutionary biology. 

As of today, a multidisciplinary team of researchers from several universities have been gathered in order to investigate and provide answers to the evolution and variation in the mating systems of certain groups of animals (birds and insects). The team is formed by Medardo Cruz López, Martín A. Serrano-Meneses (both from the Universidad Autónoma de Tlaxcala, Mexico), Raúl Cueva (Universidad Nacional Autónoma de México, Mexico), Tamás Székely (University of Bath, UK) and Clemens Küpper (University of Harvard, USA). 

Medardo Cruz López is currently an MSc. student in the Biological Sciences program, at the Centro Tlaxcala de Biología de la Conducta (Universidad Autónoma de Tlaxcala), under the supervision of Martín A. Serrano-Meneses. The model Medardo is using to investigate the selective forces that shape mating systems is the Snowy plover (Charadrius nivosus), a small shorebird that may exhibit up to three mating systems within a single population! Monogamous breeding couples remain together over a reproductive season and both provide parental care. Under polygyny, males have more than one partner and they leave females to provide all the care to the chicks. Finally, under polyandry, females have more than one partner and they leave males to provide all the care to the chicks. 

Five years ago (2006-2010) we started working on a Snowy plover population located at the Bahía de Ceuta, Sinaloa, Mexico. Every reproductive season, from 2006, Medardo and Clemens work at Ceuta and gather data on the characteristics of breeding individuals, the number of chicks they produce, how many of these survive, what parent provides most of the care, how much care the parent provides, among other data. Medardo and Clemens also obtain morphological data, such as tarsus size, wing size, body weight and photographs of the ornaments of every individual. Up to 95% of the individuals of this population have been ringed; therefore it has been possible to track the life histories of most individuals and to accurately manage data. With our datasets we aim to generate a number of research studies and to investigate in depth the traits that determine the mating systems adopted by this species.

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Biogeography of mexican dung beetles

For a long time people thought that living organisms were created in ‘centres of origin’ or creation spots, from which they moved towards the most diverse areas of the planet. In this way, people explained why kangaroos exist only in Australia, why lions are exclusive of Africa or why the highest diversity of pines is found in North America. Nonetheless, we now understand that species originate in a very different way. Charles Darwin, one of the most prominent naturalists, was interested in understanding how species originated. The presence of ostriches in Africa and that of other very similar birds in South America, the rheas, drew his attention. He reasoned that if these birds were related, then, could South America and Africa be related? The theory of continental drift and the movement of the tectonic plates gave the answer. Life and earth evolve together. Nowadays, biogeography is the result of the integration of several areas of science, such as evolution, geography and palaeontology. This multidisciplinary approach allows us to understand the distribution of living organisms on the planet. 

In 1858 Sclater, and later Wallace (1876), divided the world in several biological regions, which were characterised by the biota they contained. In 1920, Augustin de Candolle divided the world in 20 regions, which represented the different plants on the planet. Natural barriers, such as oceans, deserts, temperature changes and ecological barriers, demarcated these regions. In this way, the planet was divided into biogeographic regions that can potentially explain the distribution of current and extinct taxa. These regions also reflect the history of the planet, told by the biota itself. 

Very interestingly, Mexico seems to be the point where two types of biota diverge: the Nearctic and the Neotropic ecozones. This area of divergence was recognised by Halffter (1962) as the Mexican zone of transition. This zone exhibits a hybridisation of the biological components of both ecozones. 

Therefore, if taxa have similar distributions, one can say that they have been subjected to the same historical patterns of dispersion, so that their current distribution is the result of common histories. This is fundamental in order to understand the resulting geographic space used by the biota and is the focus of the research of Miguel Rivas Soto, a current student at the spatial analyses laboratory, at the Institute for Biology, of the National Autonomous University of Mexico. Miguel is using the distribution of several beetle taxa (Scarabaeidae) used by Halffter (1962) himself, in order to test and demonstrate the limits of the biotic regionalisation on Central- and North America. Miguel is also testing how the hypotheses of spatial homology of beetles can be contrasted and compared to those proposed for other taxa. 

Biogeographic studies are important, not only because they can account for the history of the planet, as told by living organisms, but because they can explain the occurrence of some of the biological diversity of a given region. It gives us the historical tools that we need in order to implement conservation areas, biological corridors or to plan a sustainable use of natural resources. The more taxa is used to test these hypotheses, the more we will be closer to answer the ‘what, when and where’ questions on the planet’s biodiversity; we will be more prepared when facing the global biodiversity’s crisis, caused by overexploitation, demographic growth and global weather change.

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Studies on the evolutionary ecology of leaf beetles

Leaf beetles (Coleoptera: Chrysomelidae) are a numerous group of beetles that can usually be found on the leaves of trees, bushes or weeds. They tend to exhibit attractive colours, a range of body shapes and interesting reproductive behaviours that, on the whole, make them an ideal group for ecological and evolutionary studies. Particularly, the evolution of differences in size between males and females (sexual size dimorphism) and the strategies used by the sexes in order to control the reproductive decisions of one another, have been at the centre of the research interests of Omar Sotelo.

During the 2010 reproductive season of Mexican leaf beetles (Summer – Fall), Omar worked with several species at the Reserva de la Biósfera Sierra de Huautla, a National Park located in Central Mexico, south of the State of Morelos. Omar’s preliminary results show that these beetle species exhibit a range of sexual size dimorphism (species where males are larger than females and species where females are the larger sex). With this natural variation, he will be able to test functional hypotheses that can account for the evolution and maintenance of sexual size dimorphism.

Studies like this allow us to know deep in detail the life history of these small organisms (sometimes labelled as threats to humans due to their preference to feed on commercial crops) and they enable us to produce the necessary knowledge to elaborate conservation plans. The latter are desperately needed for a wide range of habitats across Mexico.

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