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Australian Systematic Botany Australian Systematic Botany Society
Taxonomy, biogeography and evolution of plants
RESEARCH ARTICLE

Evolution of Geosiris (Iridaceae): historical biogeography and plastid-genome evolution in a genus of non-photosynthetic tropical rainforest herbs disjunct across the Indian Ocean

Elizabeth M. Joyce https://orcid.org/0000-0001-8291-8058 A B C E , Darren M. Crayn https://orcid.org/0000-0001-6614-4216 A B C , Vivienne K. Y. Lam D , Wesley K. Gerelle D , Sean W. Graham D and Lars Nauheimer https://orcid.org/0000-0002-2847-0966 A B C
+ Author Affiliations
- Author Affiliations

A Australian Tropical Herbarium, James Cook University, 14–88 McGregor Road, Smithfield, Qld 4878, Australia.

B College of Science and Engineering, James Cook University, 14–88 McGregor Road, Smithfield, Qld 4878, Australia.

C Centre for Tropical Environmental Sustainability Science, James Cook University—Cairns, 14–88 McGregor Road, Smithfield, Qld 4878, Australia.

D Department of Botany, University of British Columbia, 6270 University Boulevard, Vancouver, BC, V6T 1Z4, Canada.

E Corresponding author. Email: lizzy.joyce@my.jcu.edu.au

Australian Systematic Botany 31(6) 504-522 https://doi.org/10.1071/SB18028
Submitted: 2 May 2018  Accepted: 14 October 2018   Published: 13 December 2018

Abstract

Mycoheterotrophs, i.e. plants that acquire carbon from root-associated soil fungi, often have highly degraded plastomes, reflecting relaxed selective constraints on plastid genes following the loss of photosynthesis. Geosiris Baill. is the only mycoheterotrophic genus in Iridaceae and comprises two species in Madagascar and nearby islands, and a third recently discovered species in north-eastern Australia. Here, we characterise the plastomes of the Australian and one Madagascan species to compare patterns of plastome degradation in relation to autotrophic and other mycoheterotrophic taxa and investigate the evolutionary and biogeographical history of the genus in Iridaceae. Both examined species have lost approximately half their plastid-encoded genes and a small but significant reduction in purifying selection in retained non-photosynthetic genes was observed. Geosiris is confirmed as monophyletic, with initial divergence of the genus occurring c. 53 million years ago, and subsequent diversification occurring c. 30 million years ago. Africa (including Madagascar) is reconstructed as the most likely ancestral area of the genus, implying a major range-expansion event of one lineage to Australia after its divergence in the Oligocene. Our study has highlighted the dynamic evolutionary history of Geosiris, contributed to the characterisation of mycoheterotrophic plastomes, and furthered our understanding of plastome structure and function.

Additional keywords: degradation, heterotrophic, monocot, mycoheterotroph, plastome.


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