Evolutionary and biogeographic history of the black fly Simulium wayani (Diptera: Simuliidae) on the island of Timor

Abstract

A recently described species of black fly, Simulium wayani Takaoka and Chen, from the island of Timor was chromosomally mapped to provide insights into its evolutionary and biogeographic history. The morphologically based species status of S. wayani is supported by a suite of fixed chromosomal rearrangements and unique sex chromosomes derived primarily from a large pool of polymorphisms in the S. ornatipes complex in Australia. The banding patterns of its polytene chromosomes indicate that S. wayani is closely related to a pair of homosequential cryptic species (S. norfolkense Dumbleton and S. ornatipes cytoform A2) in the S. ornatipes Skuse complex on mainland Australia; all three species uniquely share the same amplified band in their chromosomal complement. The low level of polymorphism and heterozygosity in S. wayani, relative to Australian populations of the S. ornatipes complex, suggests few colonization events from the larger land mass.

Introduction

Natural colonization of oceanic islands is abundantly documented, and is typically influenced by factors such as distance from a source and age and size of the islands (Peck, 1996; Losos and Ricklefs, 2009). These factors also promote speciation and contribute to the unique richness of islands by restricting gene flow (Lazell, 2005). Consequently, islands feature prominently among the earth’s biodiversity hotspots (Myers et al., 2000).

Island colonization also depends on availability of appropriate habitat, such as flowing freshwater. Black flies (Simuliidae), which require running water for the larval stage, have colonized the majority of oceanic islands with flowing water (Crosskey, 1990). At some distance beyond 100 km, however, colonization by black flies across open water becomes rare and the probability of unique insular taxa on distant islands increases (Adler et al., 2005).

The Sunda Arc, a string of predominantly volcanic islands in the Indian Ocean, traces the boundary between two tectonic plates (Bird, 2003). It includes the Lesser Sundas, which are formed of an inner and an outer volcanic arc. The islands in the outer arc are less than 4 million years old and are composed of basement rock from the continental margin of the Australian plate, with uplifted coral reefs; the island of Timor, however, lacks a volcanic component (Monk et al., 1997). Timor lies to the east of Wallace’s Line and has a mixed Oriental and Australasian fauna, with high levels of endemism (Andersen et al., 2013; Takaoka et al., 2018).

The insect fauna of Timor is poorly known. The high degree (10–13%) of precinctive (sensu Frank and McCoy, 1990) birds and mammals, however, suggests that a comparable richness of unique insect species exists in the island’s threatened ecosystem, namely the Tropical and Subtropical Dry Broadleaf Forest biome of the Wallacea bioregion (Wikramanayake et al., 2002; Andersen et al., 2013). Six species of black flies are known from Timor, including the recently described Simulium (Nevermannia) wayani Takaoka & Chen, a member of the Old World S. ruficorne species group (Takaoka et al., 2018).

Our objectives were to infer the evolutionary relationships of S. wayani and provide insight into its origin on Timor. To accomplish these objectives, we analyzed the banding patterns of its polytene chromosomes.