Reconstructing plastome evolution across the phylogenetic backbone of the parasitic plant genus Cuscuta (Convolvulaceae)

Abstract

Parasitic plants have evolved to have reduced or completely lost ability to conduct photosynthesis and are usually characterized by sweeping morphological, physiological and genomic changes. The plastid genome (or plastome) is highly conserved in autotrophic plants and houses many key photosynthesis genes. This molecule is thus a useful system for documenting the genomic effects of a loss of autotrophy. Cuscuta (dodders) represents one of 12 independent transitions to a parasitic lifestyle in angiosperms. This near-cosmopolitan genus contains>200 obligate parasitic species circumscribed in four subgenera: Grammica, Pachystigma, Cuscuta and Monogynella. With respect to photosynthesis, Cuscuta is a heterogeneous group, containing both hemi- and holoparasitic members that are, respectively, partially or entirely reliant on parasitism to meet their carbon budget. Plastomes in this genus have been reported to show a substantial degree of diversification in terms of length and gene composition. Considered together with well-understood phylogenetic relationships, this genus presents an opportunity for fine-scale comparisons among closely related species of heterotrophic plants. This research documents changes in sequence composition and structure that occurred as these plants evolved along the trophic spectrum by using multiple whole-plastome assemblies from each of the four subgenera. By 'triangulating' the positions of genomic changes, we construct a step-by-s'tep model of plastome evolution across the phylogenetic backbone of Cuscuta and highlight the remarkable retention of most photosynthetic genes in these parasitic plants.