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Island gigantism or dwarfism? Phylogeny and taxonomy of Madagascar's chirping Giant Pill-Millipedes, describing newly discovered, microendemic species

REU 2010 Projects



Curator/Advisors: Dr. Petra Sierwald (Associate Curator, Zoology/Insects) and Dr. Thomas Wesener (Postdoctoral Fellow, Zoology/Insects)

Environmental Studies: Ecology and Biodiversity major
Sewanee: The University of the South

Symposium Presentation Title: Island Gigantism or Dwarfism? Phylogeny and Taxonomy of Madagascar's Chirping Giant Pill-Millipede

Symposium Presentation Abstract: The occurrence of island gigantism and dwarfism has not been extensively studied in invertebrates. On the island of Madagascar, both of these trends have been observed in the giant pill-millipede family Arthrosphaeridae (Sphaerotheriida).  In particular, the genus Zoosphaerium contains some gigantic species, while the genus Microsphaerotherium consists entirely of dwarfed forms. Within the endemic genus Sphaeromimus, only three species have been described and none are known to display gigantism or dwarfism. However, recent collections have discovered nine new populations of Sphaeromimus, whose taxonomic status was unclear. These findings include specimens from localities further north than previously known, from a cave, and from isolated, azonal rainforests, as well as the first gigantic Sphaeromimus specimensTo determine if gigantism in Arthrosphaeridae represents a plesiomorphy, or if it evolved independently in the different Malagasy genera, we constructed a molecular phylogeny. DNA of all Sphaeromimus populations, including all undescribed forms,  was extracted and fragments of the  CO1, 28S and 16s, as well as the entire 18S gene (>4000 bp in total) were sequenced. Our results indicate high microendemism within the genus, particularly in southeastern populations, with populations only 1 km apart belonging to distantly related species. Other southeastern Sphaeromimus species consist of a single population located in an extremely small area of distribution. Such sharp species boundaries are probably due to abrupt changes in habitat, including changes in soil type which suggests that speciation within the genus has occurred as a result of environmental change. Our phylogeny also reveals that our basal most species has the widest distribution and is found in the southwestern dry spiny forests, the only Sphaeromimus species adapted to such a habitat. Scanning electron microscope images taken of the antennae and endotergum of the various Sphaeromimus populations agree with our molecular data providing further support for our phylogeny. Finally, our results indicate that gigantism is an apomorphic character inside the genus Sphaeromimus and therefore has evolved multiple times on Madagascar.