| Sexual dimorphism is widespread in animal kingdom. Numerous theortical and empirical studies have demonstrated that the cause of sexual dimorphism is female-mate choice, male-male competition or both in many animals. Many animals produce a bewildering diversity of ornaments and weapons that function in female mate-choice and/or male contests. Recent research of these traits has concentrated primarily on diversity and evolution of ornaments that are involved in female mate-choice. With few exceptions (e.g., beetle horns), how or why animal weapons diverge in form remains largely unexplored. The objective of this project was to investigate the pattern of morphological evolution and diversity of elongated male chelicerae in a phylogenetic framework with respect to their functions in sexual selection of the ant-like jumping spiders from the genus Myrmarachne (Araneae: Salticidae). Myrmarachne is one of the most speciose genera in spiders, with over 200 described species worldwide. They show marked sexual dimorphism, with adult males having greatly enlarged chelicerae; like many other animal weapons such as beetle horns and crustacean claws, chelicerae do not become enlarged in males until the final moult reaching adulthood. All these are of typical characteristics of sexually selected weapons. It is also observed that the degree of sexual dimorphism is different among species. This study investigated sexual dimorphism in cheliceral size in Myrmarachne using morphology, ethology and molecular evolution. In Chapter 2, using morphological method I quantified sexual dimorphism by measuring body size parameters of eight species collected from Henan, Hubei, Hunan, Yunnan and Hainan. I found that the differences in the absolute length of chelicera and in the relative chelicera length to the body length were significant between males and females of all eight species of Myrmarachne studied. In addition, the level of this sexual dimorphism varied among Myrmarachne species. These results indicate that there exists sexual dimorphism in chelicera length Myrmarachne and that there may be a process of change on it. In Chapter 3, I used behavioural experiments to determine the function of elongated male chelicerae in female mate-choice and male-male competition in six selected species of Myrmarachne. Results showed that in some species females preferred males with longer chelicerae, but in other species elongated chelicerae had no effect on female mating preference. However, in male-male competition, males with longer chelicerae won all the contests in all the six species of Myrmarachne tested. This suggests that the elongated chelicerae could act as armament in these species of Myrmarachne. In Chapter 4,1 used five gene (18s,28s, CO1, ND1, Histone3) to reconstruct the phylogenetic tree for the species of Myrmarachne used in morphological and behavioural studies and traced sexual dimorphical data onto the phylogenetic tree to explore the evolution trend about the sexual dimorphism in chelicerae in Myrmarachne. The results from the analysis showed the relationship of length of chelicerae and degree of evolution is fluctuating. To sum up, the length of chelicera may reflect a male’s quality. The causes of sexual dimorphism may be mainly male-male contest in all species, but may also related to female mate-choice in some species. The potential cost for long chelicerae may lead the fluctuating evolution trend. This study sheds new light on our understanding of sexual selection, and it also gives us new research direction about molecular evolution of sexual dimorphism. |
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