Evolution of the male genitalia in the genus Limnebius Leach 1815, family Hydraenidae (Coleoptera)

Abstract

In the present work I analysed different aspects of the evolution of genus Limnebius (Coleoptera, Hydraenidae), including its molecular evolution using mitochondrial (COI, 16S, tRNA-leu, NAD1) and nuclear markers (18S, 28S) as well as morphological data, for which I measured more than 3000 exemplars from 123 species. The study concentrated on the shape of the male genitalia, as the species of Limnebius are extremely uniform in their external characters. In the molecular phylogeny, which included 65 species and was obtained with BEAST, there were two principal branches, coinciding with the ancient separation into two subgenera. In the one of those subgenera, Limnebius s.str., the male genitalia have an extreme variability, with from one up to seven appendages. Other than the median lobe with the spermiduct and the lateral lobes (parameres) typical of the genitalia of Coleoptera, there are up to four additional appendages, sometimes crossing each other. In the second subgenus, Bilimneus, except for one species male genitalia consist only of the median lobe. Other than differences in the male genital structure, Limnebius s.str. is more variable in body size and sexual dimorphism: in this subgenus males could be bigger or smaller than females, while among Bilimneus males are always smaller, and have no secondary sexual characters. In general a large body size is associated with larger males, presence of secondary sexual characters such as modifications of the abdomen or posterior tibias, and more complexity of the male genitalia. There are, however, some exceptions caused by secondary loss of characters, first of all, according the phylogenetic reconstructions, large body size. The size of the male genitalia is also associated with its complexity, but less than with body size, and with more random evolutionary changes. The reconstruction of the ancestral body size for each of the subgenera show that in the origin it was larger in Limnebius s.str. than in Bilimneus, and this could be the reason for the differences in evolution. This reason may not be sufficient, but we have no other evidence for the differences between the evolution of the two subgenera. Differences could also be due to the structure of the parameres, which in Limnebius s.str. are separated from the medial lobe, but we have no precise data to support this hypothesis. The complexity of the male genitalia in Limnebius s.str. evolved in two different forms, with different consequences for the morphological changes: in one (L. parvulus group) its seems that size of the body and genitalia increased before the complexity, and in the other (L. nitidus group) it seems that the complexity is less dependent on the size. Complexity was secondarily lost in one of the subgroups, which could be an indication of character saturation. Other than these differences there are also structural difference in complexity, formed by structures completely different between the two groups. Their homology is possible, but it is not obvious even in the histological serial sections.

En la presente tesis he analizado varios aspectos de la evolución del genero Limnebius (Coleoptera, Hydraenidae): molecular (utilizando los marcadores mitocondriales COI, 16S, tRNA-leu, NAD1 y los nucleares 18S y 28S) de 65 especies y morfológica, utilizando mas de 3,000 ejemplares de 123 especies. En el árbol molecular hay dos ramas principales que coinciden con la antigua separación en dos subgéneros: Limnebius s.str., cuya genitalia masculina tiene una enorme variabilidad, con desde uno hasta 7 apéndices; y Bilimneus, que con la excepción de una especie sólo tienen la pieza principal por donde pasa el espermiducto. Limnebius s.str varía más en el tamaño del cuerpo y en el dimorfismo sexual: los machos pueden ser mas pequeños o mas grandes de las hembras, mientras que en Bilimneus los machos siempre son más pequeños y sin caracteres sexuales secundarios. El cuerpo grande está asociado a machos más grandes, presencia de caracteres sexuales secundarios, como modificaciones en el abdomen y en las tibias posteriores, y una mayor complejidad de la genitalia masculina. Hay pérdidas de complejidad secundarias, sobre todo del tamaño del cuerpo. El tamaño de la genitalia masculina esta asociado con su complejidad, pero menos que el tamaño del cuerpo, y cambia de forma más aleatoria. La reconstrucción del tamaño del cuerpo ancestral muestra que en su origen Limnebius s.str. era más grande que Bilimneus, lo que podría ser la razón de las diferencias en su evolución, así como la estructura de los parámetos, que en Limnebius s.str. están siempre más o menos separados de la pieza principal. La complejidad de la genitalia en Limnebius s.str. evolucionó de dos formas distintas, con diferentes consecuencias para los cambios morfológicos: en una (grupo de L. parvulus) el tamaño de cuerpo y la genitalia parece que aumentan antes que la complejidad, y en la otra (grupo de L. nitidus) parece que la complejidad depende menos del tamaño, aunque en uno de los subgrupos se pierde, lo que puede ser una indicación de su saturación. Hay estructuras aparentemente completamente diferentes entre estos dos grupos, cuya homología es posible pero no se puede apreciar en cortes histológicos.