Molecular Ecology and Evolution
Wicke, Susann , Mueller, Kai , Quandt, Dietmar , Wickett, Norman , dePamphilis, Claude , Schneeweiss, Gerald .
Broomrape plastid genomes reveal distinct patterns of functional and physical gene deletion under relaxed selective constraints.
Non-photosynthetic plants often possess highly reduced plastid chromosomes compared to their autotrophic relatives. As a result of the loss of autotrophy, genes involved in photosynthesis as well as protein-coding housekeeping genes are lost convergently in several unrelated lineages. Nevertheless, even 20 years after the first parasitic plant plastome had been sequenced, little is known about how functional and structural genome reduction takes place. In the present study,we trace the complex history of genome reduction in a group of closely related parasites of the broomrape family (Orobanchaceae). This group represents a wide array of intermediates in the process of plastome reduction that allows us to determine elementary patterns of deletion of dispensable DNA-fragments. To this end, we sequenced the plastid genomes from several photosynthetic and non-photosynthetic parasitic broomrape species. We thoroughly analyzed the structural evolution with respect to co-linearity, gene content, and functionality of genes. Using reconstructions of plastome rearrangements and ancestral gene content, we assessed molecular evolutionary patterns of gene loss and the deletion of DNA-segments under relaxed selective constraints. We provide convincing evidence that functional plastome reduction occurs in the early stages of heterotrophy, suggesting that the establishment of obligate parasitism can be viewed as the major, relaxed selective constraint en route to becoming non-photosynthetic. Increasing amounts of plastid repetitive DNA in parasites eventually entail increased rates of improper and/or illegitimate recombination leading to the eventual deletion of plastid-chromosomal fragments. Amongst others, our analyses reveal that the functional and physical plastome reduction coincides with a measurable increase in A/T-content in both coding and non-coding plastid DNA-fractions. Furthermore, we demonstrate that proximity to functionally relevant groups is the primary determinant of longevity and retention of dispensable genes and gene regions. By using an unprecedented comparative genomic approach, this study can provide detailed insights into the complex patterns of reductive evolution of plastomes under relaxed selective pressures.
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1 - University of Muenster, Institute for Evolution and Biodiversity, Huefferstr. 1, Muenster, 48149, Germany
2 - University of Bonn, Nees Institute for Biodiversity of Plants, Meckenheimer Allee 170, Bonn, 53115, Germany
3 - Chicago Botanic Garden, Department Of Ecology & Evolutionary Biology, 5108 N Ashland Ave, Apt 2, Chicago, IL, 60640, USA
4 - Pennsylvania State University, Department Of Biology, 101 LIFE SCIENCES BUILDING, UNIVERSITY PARK, PA, 16802, USA
5 - University of Vienna, Department of Systematics and Evolutionary Botany, Rennweg 14, Vienna, 1030, Austria
Presentation Type: Oral Paper:Papers for Topics
Location: Union B/Hyatt
Date: Monday, July 9th, 2012
Time: 2:00 PM