Quantifying metamorphosis and the importance of the transitory stage

Braig, Florian (2024): Quantifying metamorphosis and the importance of the transitory stage: examples from meiurans and holometabolans. Dissertation, LMU München: Faculty of Biology

Abstract

The group Crustacea sensu lato comprises some of the most characteristic and numerous arthropodans on the planet, such as crabs, copepods, beetles, and bees. The group of crabs, Meiura, consists of Anomala (or Anomura) and Brachyura, commonly called true and false crabs. Together both groups amount to ca. 10,000 globally distributed species, though the species diversity is not equally distributed between the two groups. The similarly charismatic holometabolan insects include about 800,000 formally described species, representing a large proportion of species diversity of our planet. Both groups are often described as evolutionary successful, holometabolans within insects and crabs within decapod crustaceans, respectively. In both groups this evolutionary success is often attributed, at least in part, to the ecological differentiation of development. Post-embryonic development in both groups is divided into a first larval phase for dispersal and growth in a different habitat from the adult. This phase is followed by a transitory larval phase, which is then followed by the adult phase. This developmental pattern is characterized by metamorphosis, a drastic change of morphology and ecology in a short. The development of both groups has been studied extensively, but its evolutionary trajectory is still discussed in both groups. The lack of larvae in the fossil record of meiurans and the low number of well-preserved pupae in holometabolans makes the study of the evolution of this specialized form of development difficult. Recent advances in increasing the record of fossil larvae have not been fruitful. Therefore, I compared the general post-embryonic development in the groups Meiura and Holometabola to find similarities and differences, to better understand evolutionary trajectories and selective advantages of this form of development. I furthermore both quantified and classified development using geometric morphometric approaches to obtain quantitative morphological data on the development. Development in both groups can be indeed quantified and shows large morphological disparities between larvae and adults of groups undergoing metamorphosis, as I could show in three studies. In one study I could show that developmental phases can be classified based on their appearance, which also revealed the morphological disparity that juvenile and adult crabs have to each other. In analysing the developmental patterns respectively in seven studies, I found similarities in the overall discretization of development. Furthermore, caenogenesis and heterochrony, delays in the first larval phase and accelerations in the transitory larval phase have shaped development in both groups. The division of tasks was not equal between both groups, but I could outline a potential imbalance in the division of tasks within development of both groups, that may have led to phases with less tasks to diversify more. These changes have overall made the development of both groups more metamorphic and more discrete. This discretization is assumed to provide further selective benefits in Meiura and Holometabola. I could also show that the megalopa phase, the transitory phase of meiurans, is plesiomorphic for the group and potentially phylotypic. The pupa in holometabolans on the other hand is not plesiomorphic but may be phylotypic as well. Therefore, only the pupa provides an evolutionary new form increasing the diversity of the group. In Meiura the diversity is created by the new adult forms. However, the different evolutionary origins create the impression of convergent evolution towards a discretized development with a transitory larval phase in the middle.

• Item Type: Theses (Dissertation, LMU Munich)
• Subjects: 500 Natural sciences and mathematics; 500 Natural sciences and mathematics > 570 Life sciences
• Faculties: Faculty of Biology
• Language: English
• Date of oral examination: 13. August 2024
• 1. Referee: Haug, Joachim
• Signature of the printed copy: 0001/UMC 32190