Relationship Between the Weights of Seed Beetles of the Genus
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www.nature.com/scientificreports OPEN Relationship between the weights of seed beetles of the genus Megacerus Fåhraeus, 1839 Received: 11 January 2019 Accepted: 22 May 2019 (Coleoptera: Chrysomelidae: Published: xx xx xxxx Bruchinae) and their host seeds of the family Convolvulaceae A. Canto 1, R. Rodríguez1 & E. Reyes-Novelo 2 We studied seeds from a set of plant species from the Convolvulaceae family. Seeds collected from natural populations and infested with beetles of genus Megacerus were monitored until the beetle emergence. We analyze the relationship between body weight of beetles and seed weight of host plants, and its connection with between-species diferences and sexual dimorphism. The results show that diferences in the scaling of body weight of beetles are associated with sexual dimorphism. For the same species of beetle, the females tend to have heavier bodies than the males. Diferences between host plants species in the weight of seeds are related to diferences in the body weight Megacerus species, resulting in a distinctive pattern of seed infestation across hosts. Small-sized (lighter) species of beetles tended to infest small-sized (lighter) seed species and, correspondingly, heavier beetles species tended to do it in heavier seed species. Mechanisms of female oviposition preferences may be involved to generate that pattern. In general, the beetle weight showed an asymptotic relation with the host seed weight. The greater the weight of the seed, the greater the weight of adult beetle was. However, the proportion in weights reaches an asymptotic value probably because beetles reached the maximum possible weight for their species. We conclude that the process of specialization in the seed-beetle assemblage studied is infuenced by intrinsic traits of the species involved in the interaction (beetles and seeds) and by diferences between sexes in their sexual-allocation paths. In the seed-beetle interactions, the size of the seed is a direct determinant of the quality of the adult beetles. Te availability of food within the seeds is essential for the adaptation of individuals since the acquisition of nutrients during the larval stages defnes the beetles’ performance in adult life1,2. Seeds have an embryo, cotyledons and endosperm that are rich in carbohydrates and proteins and together provide sufcient resources for the full development of the larvae. However, depending on the environment and natural history, seeds vary in the amount of resources they contain, both among individual plants and among species3, thus resulting in variations in their quality as a food source for the seed beetles4. Te quality of the seed components strongly infuences the adaptation and reproductive strategies of the insects because the body size of the individuals is modifed based on the accumulation of resources, and body size is a key feature for individual performance refected in development and survival5. For example, beetles that emerge from larger seeds tend to be larger adults that survive longer, have a greater ability to evade predators and a better immune capacity than smaller-sized beetles1,6,7. Variations in seed weight directly afect fertility of individuals and can lead to diferentiation in size between males and females. Te evolutionary specialization of the reproductive roles between males and females produces 1Centro de Investigacion Cientifca de Yucatan, A.C., Merida, Mexico. 2Centro de Investigaciones Regionales Dr. Hideyo Noguchi, Universidad Autonoma de Yucatan, Merida, Mexico. Correspondence and requests for materials should be addressed to A.C. (email: [email protected]) SCIENTIFIC REPORTS | (2019) 9:8438 | https://doi.org/10.1038/s41598-019-44761-8 1 www.nature.com/scientificreports/ www.nature.com/scientificreports sexual size dimorphism diferences regardless of the proportional efect of the seed size on body size of beetles. Tis is because, weight gain in females is mostly invested in fertility (to lay more eggs), while resource acquisi- tion in males is invested mostly in structures associated with display for mating. Larger females will have higher fecundity but males with larger display structures will have more opportunity to attract females. Te fnal result of the diferential investment of resources between females and males is that females will be heavier than males regardless of the weight of the host seed5,8,9. Diferences in the weight of seeds between the plant species that acts as the seed host is a trait that can infuence the fnal size of the individuals4,9. In the present work, a particular set of species of seed beetles of the genus Megacerus (Coleoptera: Chrysomelidae: Bruchinae) and seeds of the family Convolvulaceae of southeastern Mexican region are studied. Te larval stages of this group of beetles depend mainly on the seeds of this family of plants to complete their development10,11. Te beetle and plant species are native to Mexico and distributed in practically all environments of the Yucatan Peninsula. Tus far, 11 Megacerus species have been reported in 10 plant species for the region12,13. Te distribution of beetles follows the distribu- tion of the host plants, which are relatively abundant species that fower simultaneously in masse during the cold season14. Tis system of interactions between beetle and seed species is conducive to exploring the efect of the seed characteristics on the beetles that prey on diferent seed species since the same species of beetle can invade dif- ferent seed species with diferent sizes and one seed species can vary in size and be invaded by diferent species of beetles that vary in size. Additionally, seed and beetle species can be identifed relatively easily at the species level. Megacerus species present an evident sexual dimorphism in antennae and eyes that makes them recognizable from emergence. Te purpose of the study is to gain knowledge about the underlying ecological specialization between beetle species and their host seed species. We hypothesize that the relationship of weights between seeds and adult beetles in the Convolvulaceae-Megacerus interactions, is modifed by ecological components such as variation in the seed weight within species of host plants and sexual dimorphism in the body size of beetles, as well as diferences in the seed weight of host plant species. We expect sex-biased diferences in the body weight scaling between males and females, diferences in the body weight scaling within Megacerus species that use host seeds of diferent weight, and that the body weight in beetles of the same species changes proportionally to the weight of their host seed. Materials and Methods Study area. Within the Yucatan region, four areas were selected from north to south that host wild popula- tions of the most frequent species of Convolvulaceae. Te areas were Conkal (21°04′24″N; 89°31′15″W), Yaxché (21°07′02″N; 89°38′08″W), Xcunyá (21°07′58″N; 89°36′47″W) and Kiuic (20°03′05″N; 89°33′55″W), which have similar temperature and precipitation conditions, with an average monthly temperature of 26 °C and average annual precipitation of 1096 mm15. Te predominant type of vegetation in the areas to the north is lowland decid- uous forest and to the south is mid-elevation evergreen forest, and the altitude range varies from 9 masl in the northern areas to 90 masl in the southern areas16. Seed beetles. Te beetles of the genus Megacerus are exclusive to America and belong to the monobasic tribe Megacerini within the subfamily Bruchinae associated to Convolvulaceae family10,11,17. Tey present four instars, from larvae to pupa whose development depends entirely on the seeds18. In the study area, larvae of the Megacerus species develop inside seeds of plant species of the genera Ipomoea and Merremia (Convolvulaceae). Male and female adults of Megacerus can ofen be found feeding on the nectar and pollen of the fowers of these plant genera. As far as is known, the females deposit their eggs on the immature fruits, and afer few days (four to fve days in Megacerus discoidus18), the larvae emerge and pierce the plant tissue until they reach the seeds and embed themselves. Tey then feed on the endosperm and seed embryo until reaching the imago stage (adults recently emerged from pupal cocoon). Plants studied. In each study location, 5–10 collection points were selected whose selection criterion requirements were populations of each of the following species of Convolvulaceae: Ipomoea triloba, Ipomoea hederifolia, Ipomoea nil and Merremia aegyptia. The species belong to the tribe Ipomoeeae and sister tribe Merremieae19, and their populations are generally found in sunny environments with open vegetation com- posed of herbaceous or low scrub. Tese plants are annual vines that grow massively and whose fowering occurs towards the cold season between September and October. Te fowers are showy and hermaphroditic and have a superior ovary with two or four deeply grooved carpels, locules equal in number to carpels and four ovules in total, with two per carpel. From our feld observations, the oviposition of females during the fowering season of Merremia and Ipomoea species, occurs on the external surface of the sepals of the calyx which contains the fower ovaries (the future fruit with seeds). Te fruits are longitudinally dehiscent capsules in four segments with seeds that contain a cartilaginous homogeneous endosperm and a large axillary embryo that occupies most of the seed, with thin cotyledons that are extensive and folded multiple times20. Seed dispersal is from November to March, which is favorable for collecting the fruits since the larvae that have managed to embed themselves within the seeds are completing their development13. Figure 1 shows the type of seed of each species. Seed collection and emergence of beetles. Seed collection was carried out during three consecu- tive years (2008, 2009, 2010) during the seed dispersal stage, and at each collection point, 3–5 plants spatially separated by at least 20 meters were selected.