Host‐driven divergence in the parasitic plant Orobanche minor Sm. (Orobanchaceae)

Abstract

Many parasitic angiosperms have a broad host range and are therefore considered to be host generalists. Orobanche minor is a nonphotosynthetic root parasite that attacks a range of hosts from taxonomically disparate families. In the present study, we show that O. minor sensu lato may comprise distinct, genetically divergent races isolated by the different ecologies of their hosts. Using a three-pronged approach, we tested the hypothesis that intraspecific taxa O. minor var. minor and O. minor ssp. maritima parasitizing either clover (Trifolium pratense) or sea carrot (Daucus carota ssp.gummifer), respectively, are in allopatric isolation. Morphometric analysis revealed evidence of divergence but this was insufficient to define discrete, host-specific taxa. Intersimple sequence repeat (ISSR) marker-based data provided stronger evidence of divergence, suggesting that populations were isolated from gene flow. Phylogenetic analysis, using sequence-characterized amplified region (SCAR) markers derived from ISSR loci, provided strong evidence for divergence by clearly differentiating sea carrot-specific clades and mixed-host clades. Low levels of intrapopulation SCAR marker sequence variation and floral morphology suggest that populations on different hosts are probably selfing and inbreeding. Morphologically cryptic Orobanche taxa may therefore be isolated from gene flow by host ecology. Together, these data suggest that host specificity may be an important driver of allopatric speciation in parasitic plants.

Parasitic plants have increased rates of molecular evolution across all three genomes

“…Chloroplast genes in different species of Orobanche (broomrape), a non-photosynthetic root parasite, show variable rates of decay into pseudogenes [23], and parasitic plants show different degrees of loss of genes from the plastid genome [20]. Some parasitic plants retain low levels of photosynthetic activity, insufficient to live independently, but enough to tide them over while waiting to find a suitable host [24].…”

“…So selection could raise the substitution rate in a wide variety of genes involved in successful exploitation of hosts, including genes that might be considered “house keeping genes” [5]. The arms race between host and parasite may account for the divergence of parasitic lineages onto different host plant populations, such that parasites can be more successfully grown on their local host plants than those from another population [23]. …”

The Genetic Structure of WildOrobanche cumanaWallr. (Orobanchaceae) Populations in Eastern Bulgaria Reflects Introgressions from Weedy Populations

“…This was an unexpected result, since, within a number of largely self-pollinated parasitic plant species, host specificity has been found as a mechanism of accelerating isolation and subsequently genetic divergence among populations, for example, in Orobanche minor Sm. [52–54], Striga asiatica [23], and S. gesnerioides [24, 25]. Conversely, Vaz Patto et al [20] studied the genetic structure of five Moroccan O. foetida populations, four of them parasitizing wild plants ( Scorpiurus muricatus L. and Ornithopus sativus Brot.)…”

Specific developmental pathways underlie host specificity in the parasitic plant Orobanche

“…gummifer) respectively in northern Europe. 5 Recently, to explore the potential for host specificity to drive this genetic divergence, we used reciprocal-infection experiments to quantify host specificity in terms of (1) relative parasite success during early development; (2) fitness in terms of biomass, and (3) anatomy of the host-parasite interface. The results from this investigation demonstrate that genetic races occurring naturally on red clover and sea carrots show distinct developmental pathways on their natural and alien hosts.…”

“…24,25 and Orobanche spp. 5,6,26 Given the predominance of host specificity among parasitic angiosperms, and our rudimentary understanding of the evolution of host-parasite interactions, it is likely that this process may have been an underestimated driving force in the evolution of this important group of plants. determining factor; later stages of parasite development and parasite biomass were however better on red clover for this race.…”