Research Projects

How does geographic isolation shape genomic divergence, ecological interactions, and phenotypic evolution? I integrate fieldwork, natural history collections, genomics, AI, and open-source software to find out.

Systematics, Island Speciation, & Host-Parasite Interactions

Islands and mountains as natural laboratories of evolution

The Indo-Australian Archipelago (IAA) combines marine barriers, ancient paleo-islands, and steep mountains, a system where isolation has repeatedly driven diversification. During the Pleistocene, when sea levels were lower, Sumatra, Java, and Borneo joined mainland Asia as Sundaland, while Sulawesi, the Moluccas, and the Lesser Sundas stayed isolated by deep water. Like the Sunda Shelf, the Sahul Shelf was continuous landmass that connected the Australian continent to New Guinea during this time. Within islands, mountains act as sky islands. Many of these regions remain poorly surveyed. This exceptional geographic complexity provides a natural laboratory for disentangling how different forms and histories of isolation generate and maintain biodiversity. I ask how geographic isolation shapes genomic divergence, ecological interactions, and phenotypic evolution.

Pleistocene low sea-level boundaries

Pleistocene low sea-level boundaries of the Indo-Australian Archipelago Map of Southeast Asia and northern Australia. Lowering sea level by 40, 80, and 120 metres exposes the Sunda Shelf, joining Sumatra, Java, and Borneo to mainland Asia as Sundaland, and the Sahul Shelf, joining New Guinea to Australia. Sulawesi and the other islands of Wallacea stay isolated by deep water. Wallace's, Huxley's, Weber's, and Lydekker's lines can be shown or hidden. Wallace's line. Wallace (1863): between Bali and Lombok, Borneo and Sulawesi. Huxley's line. Huxley (1868): like Wallace's, but placing the Philippines (except Palawan) east of the line. Weber's line. Weber (1902): the line of faunal balance between Asian and Australian animals. Lydekker's line. Lydekker (1896): the western edge of the Sahul Shelf. 0 400 800 km

Sea level: today

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As Pleistocene ice sheets grew, sea level fell by up to ~120 m, most recently at the Last Glacial Maximum about 20,000 years ago. The shallow Sunda Shelf joined Sumatra, Java, and Borneo to Asia as Sundaland, and the Sahul Shelf joined New Guinea to Australia, while Sulawesi and the rest of Wallacea stayed surrounded by deep water. Exposed shelves from ETOPO1 bathymetry; line placements are approximate.

In situ diversification of the Bunomys clade on Sulawesi

Lenomys meyeri, an endemic rat of Sulawesi

My main study system is the most speciose rodent clade on Sulawesi: the Bunomys clade. I study its systematics and in situ diversification. Integrating morphology, multi-marker Sanger sequencing, and ultra-conserved elements, we revealed taxonomic problems in the group and a complex interplay between paleo-islands, tectonic plates, and mountain formation in promoting its diversification, with mixed support for the Area of Endemism paradigm on Sulawesi.

This work builds on 13 years of collaborative field expeditions across more than 20 remote mountain systems in Southeast Asia: over 20 field-collecting expeditions contributing more than 8,000 voucher specimens and 30,000 specimen parts to natural history collections. Together with collaborators, this fieldwork has contributed to the description or revision of more than 24 mammal species. Beyond Bunomys, I support collaborative studies of shrew diversification on Sulawesi, Java, and Sumatra. I received the 2023 Murie Family Conservation Award from the American Society of Mammalogists for the significance of this work in conserving mammals and their habitats.

Systematic and Divergence times of the Bunomys clade

2.58 Ma Outgroups (not shown) 3.04 Ma (95% HPD 2.57–3.51), posterior 0.61 2.81 Ma (95% HPD 2.34–3.25), posterior 0.99 1.25 Ma (95% HPD 0.81–1.70), posterior 1.00 2.43 Ma (95% HPD 2.04–2.84), posterior 1.00 1.39 Ma (95% HPD 1.12–1.67), posterior 1.00 0.75 Ma (95% HPD 0.51–0.99), posterior 1.00 1.29 Ma (95% HPD 1.05–1.56), posterior 0.94 0.96 Ma (95% HPD 0.70–1.23), posterior 0.99 1.14 Ma (95% HPD 0.92–1.38), posterior 0.99 1.00 Ma (95% HPD 0.77–1.24), posterior 0.74 2.32 Ma (95% HPD 1.95–2.70), posterior 0.93 2.19 Ma (95% HPD 1.82–2.56), posterior 0.47 2.12 Ma (95% HPD 1.76–2.51), posterior 0.32 1.65 Ma (95% HPD 1.28–2.03), posterior 1.00 2.25 Ma (95% HPD 1.88–2.63), posterior 0.41 1.65 Ma (95% HPD 1.31–1.99), posterior 1.00 1.88 Ma (95% HPD 1.54–2.23), posterior 1.00 1.45 Ma (95% HPD 1.15–1.75), posterior 1.00 1.33 Ma (95% HPD 1.04–1.63), posterior 0.49 Halmaheramys bokimekot Halmaheramys wallacei Bunomys sp. nov. Bunomys torajae Bunomys chrysocomus Bunomys prolatus Bunomys penitus Bunomys andrewsi Bunomys coelestis Frateromys fratrorum Gen. nov. sp. nov. Eropeplus canus Lenomys meyeri Taeromys dominator Taeromys celebensis Taeromys taerae Taeromys sp. nov. Taeromys calitrichus Taeromys punicans Pliocene Pleistocene Holocene, the past 11,700 years Holocene 0 0.5 1 1.5 2 2.5 3 3.5 Ma
  • Bunomys
  • Taeromys
  • Frateromys, Lenomys, Eropeplus, gen. nov.
  • Halmaheramys (Halmahera)
Time-calibrated tree of 19 species from 55 individuals and five loci, estimated with BEAST 2. Halmaheramys of the northern Moluccas is sister to all Sulawesi lineages, with the divergence date estimated at 2.81 Ma (95% HPD 2.34–3.25), near the start of the Pleistocene. Bars span the 95% HPD of each node age; filled nodes have posterior probability ≥ 0.95. Outgroups are pruned. Learn more in Handika et al., 2021.

Sulawesi as an island of islands: A brief overview

Sulawesi is a complex island with a rich geological history. It has been shaped by ancient paleo-islands, tectonic plate movements, and mountain formation. Together, these factors have shaped the Bunomys radiation. Above about 1,000 m, many species are confined to a single mountain, while others have widespread distributions that span multiple mountains and across the island.

Four sky islands in the central core

Four sky islands in Sulawesi's central core A schematic north–south profile through Rorekatimbo, Torompupu, Gandangdewata, and Latimojong, spaced by their real distances (43 km, 175 km, 99 km). Taeromys dominator occurs on all four mountains. Above 1,000 m, montane lineages are confined to one or two summits: Eropeplus canus has a different haplogroup on Rorekatimbo and Torompupu but shares one between Gandangdewata and Latimojong; Bunomys torajae differs between Gandangdewata and Latimojong; Bunomys penitus WC-B is shared by Rorekatimbo and Torompupu. 1,000 m 2,000 m 3,000 m 1,000 m: lowland | montane Rorekatimbo 2,610 m 3 montane taxa Torompupu 2,495 m 4 montane taxa Gandangdewata 3,074 m 4 montane taxa Latimojong 3,478 m 2 montane taxa ~350 m ~500 m 43 km 175 km 99 km

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Select a taxon to see where it lives

Montane

Widespread

Orange dots are Taeromys dominator, the only taxon on all four mountains. Select a taxon to show its populations and elevation range.

Distance alone does not predict shared ancestry. Rorekatimbo and Torompupu, 43 km apart, have different E. canus haplogroups yet share B. penitus WC-B. Mts. Gandangdewata and Latimojong, 99 km apart, on the other hand, share an E. canus haplogroup but not B. torajae. The peaks in the schematic profile are drawn according to their present-day summit heights.

Across the island

Sulawesi is often divided into seven areas of endemism (AoEs) separated by narrow boundary zones. Taeromys dominator, a large endemic rat found across the island and from lowlands to high elevations, shows how messy the system is: one ancestry cluster spans mountains in every area of endemism, populations in the southeast carry a distinct cluster, and several individuals are admixed, so the AoE boundaries are only partly respected.

Sulawesi: areas of endemism and Taeromys dominator ancestry Shaded relief map of Sulawesi with dark bands marking the boundary zones between seven areas of endemism. Pie charts show the ancestry proportions of 29 Taeromys dominator individuals from 12 localities across four genetic clusters. The base map can show land and shallow sea 4, 3, 2, or 1 million years ago, redrawn from Nugraha and Hall (2018), against today's coastline. Katopasa Boganinani Ambang DuaSaudara Buliohuto Lawaki Bawakaraeng Balease Gandangdewata Latimojong Rorekatimbo Salutiwo 0 100 200 km

Ancestry clusters (K = 4)

  • Cluster 1
  • Cluster 2
  • Cluster 3
  • Cluster 4

Boundary zones between areas of endemism

  • Cluster 2 occurs in every area of endemism sampled, from the southwest peninsula to the northeastern tip.
  • Cluster 4 dominates the southeast (Lawaki), with a trace in one west-central individual.
  • Cluster 1 and Cluster 3 occur at scattered sites, and several individuals are admixed, some sharing a mountain with unadmixed neighbours.

Each bar is one individual

Hover or tap a bar or pie, or tab to a pie, to see the specimen, locality, and ancestry.

Choosing K

0 1 2 K=2 K=3 K=4 K=5 K=6 K = 2: DLK2 = 1.393 K = 3: DLK2 = 0.379 best K = 4 K = 5: DLK2 = -0.06 K = 6: DLK2 = 0.557

PopCluster's DLK2 statistic, the second-order change in likelihood between successive K, peaks at K = 4 (K = 1–7, five replicates each).

Ancestry of 29 T. dominator from 12 localities, estimated from ultra-conserved element (UCE) SNPs with PopCluster at K = 4, the best-supported K by DLK2. Each pie is one individual; individuals from the same locality are fanned out around it. The bands mark the boundary zones between areas of endemism.

Parasites and microbiomes: collaborations and next steps

The evolutionary history of the Bunomys clade also provides a framework for asking whether parasites co-evolve with their hosts or cross species boundaries. Work led by Kevin Rowe's lab at Museums Victoria, Australia, on which I am a co-author, documented native and introduced Trypanosoma lineages in Sulawesi rodents, with high prevalence in several Bunomys species and infections from village edges to remote montane forests, peaking above 2,000 m. The Esselstyn lab studies the gut microbial communities of Sulawesi shrews and rodents.

Building on these collaborations, parasite evolution is a direction I hope to pursue by integrating host population genomics, parasite phylogenies, environmental data, and single-cell sequencing. This framework would test whether parasite diversification parallels host history or reflects host switching and local adaptation, while also examining how these processes shape host-cell tropism and immune responses. Single-cell or single-nucleus sequencing of infected tissues could link host and parasite genetic variation to the specific cell types and transcriptional states associated with infection. To lay the groundwork, NAHPU records parasites collected in the field alongside their host specimens and tracks their screening, keeping parasite identity, infection status, and molecular data linked to the host from which they originated.

Sampling and data

  1. 1Field collection

    Collect host specimens and associated parasites with environmental metadata (location, habitat, climate).

  2. 2Sample processing

    Host tissue for genomics and single-cell, parasites for genomics and transcriptomics, and host and parasite IDs.

  3. 3Data generation

    • Host population genomics Whole genome or targeted sequencing
    • Parasite genomics Phylogenies and population structure
    • Single-cell or single-nucleus RNA-seq Host and parasite transcripts in infected tissues
    • Environmental data Climate, land use, and altitude
  4. 4Linked in NAHPU

    • Links each parasite to its host specimen and collection metadata
    • Tracks screening results, sequencing data, and tissue use
    • Maintains the chain from field specimen → molecular data → analyses

Analyses

  • A. Host population genomics

    Host genetic structure and history

    • Population 1
    • Population 2
    • Population 3
  • B. Parasite phylogenetics

    Parasite evolutionary relationships and host associations

    Tips colored by host population

  • C. Single-cell transcriptomics

    Host and parasite cells, cell types, and infection states

    • Host immune
    • Host epithelial
    • Other host
    • Parasite
    • Infected host
  • D. Environmental context

    Environmental drivers of host and parasite variation

    • Population 1
    • Population 2
    • Population 3

Key questions

  1. 1 Do parasite lineages preferentially occupy particular host cell types?
  2. 2 Do parasite lineages elicit different cellular responses in different host populations or species?
  3. 3 Do recently host-switched parasites show different cell-type tropism or transcriptional states from long-associated parasites?

Outcomes

  • Test whether parasite diversification parallels host history or reflects host switching and local adaptation.
  • Identify host cell types and immune pathways targeted by different parasite lineages.
  • Link evolutionary patterns to environmental drivers and cellular mechanisms of infection.
Proposed study design. Field samples linked in NAHPU support four analyses that together address host–parasite evolution from populations to cells.
Speciation and systematics:
Handika et. al. ( , , )
Chipps et. al. ( )
Demos et. al. ( , )
Esselstyn et. al. ( , , )
Giarla et. al. ( )
Nations et. al. ( )
Host–parasite and microbiome:
Adams et al. ( )
Mursyid et. al. ( )
Swanson et. al. ( )
Winterhoff et. al. ( )