Abstract
To elucidate the speciation mechanisms prevalent within hotspots of biodiversity, and the evolutionary processes behind the rise of their species-rich and endemic biota, we investigated the phylogeny of the giant fire-millipede genus Aphistogoniulus Silvestri, 1897, a Malagasy endemic. This study is the first comprehensive (molecular and morphological) phylogenetic study focusing on millipede (class Diplopoda) speciation on Madagascar. The morphological analysis is based on 35 morphological characters and incorporates ten described as well as two newly described species (A. rubrodorsalis n. sp. and A. jeekeli n. sp.) of Aphistogoniulus. The molecular analysis is based on both mitochondrial (COI and 16S), and nuclear genes (complete 18S rDNA), together comprised of 3031 base pairs, which were successfully sequenced for 31 individual specimens and eight species of Aphistogoniulus. In addition to the null-model (speciation by distance), two diversification models, mountain refugia and ecotone shift, were discovered to play a role in the speciation of soil arthropods on Madagascar. Mountain refugia were important in the speciation of the A. cowani clade, with three species occurring in the Andringitra and Ranomafana Mountains in the southeast (A. cowani), the Ambohijanahary and Ambohitantely Mountains in the mid-west (A. sanguineus), and the Marojejy Mountain in the northeast (A. rubrodorsalis n. sp.). An ecotone shift from the eastern rainforest to the unique subarid spiny forest of Mahavelo was discovered in the A. vampyrus - A. aridus species-pair. In the monophyletic A. diabolicus clade, evidence for divergent evolution of sexual morphology was detected: species with greatly enlarged gonopods are sister-taxa to species with normal sized gonopods. Among the large-bodied Spirobolida genera of Madagascar, Colossobolus and Sanguinobolus were found to be close sister-genera to Aphistogoniulus. Forest destruction has caused forest corridors between populations to disappear, which might limit the possible resolution of biogeographic analyses on Madagascar.
Highlights
Madagascar is one of the world’s centers of endemism and is considered a biodiversity hotspot [1]; with its highly diverse ecosystems, it mirrors a small continent rather than an island
Nomenclatural Acts The electronic version of this document does not represent a published work according to the International Code of Zoological Nomenclature (ICZN), and the nomenclatural acts contained in the electronic version are not available under that Code from the electronic edition
(1) The A. cowani clade, clade D in our cladogram (Fig. 2), includes species restricted to areas with an elevation higher than 900 m
Summary
Madagascar is one of the world’s centers of endemism and is considered a biodiversity hotspot [1]; with its highly diverse ecosystems, it mirrors a small continent rather than an island. Located off the coast of eastern Africa, until 88 million years ago Madagascar formed a landmass with India [2]. The Madagascar-India landmass separated from continental Africa in the early Mesozoic era, 158–160 million years ago [3], land bridges towards Antarctica might have existed up to a more recent date [4]. The isolation of Madagascar makes the island an ideal model region for the study of diversification in species, because unlike on most continents, introgression or admixture events with lineages that evolved outside the area can be almost ruled out [12]. Almost exclusively on vertebrates, present different hypotheses of diversification mechanisms on Madagascar (see overview in [12]), including mountain refugia [13,14], retreatdispersal watersheds [15], and river barriers [16]. Some speciation patterns might already be artificially modified due to habitat destructions by humans, especially in the transitional zones [17]
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