Integrating genetics, morphology, and fungal host specificity in conservation studies of a vulnerable, selfing, mycoheterotrophic orchid

Abstract

Mycoheterotrophic plants derive most or all carbon and nutrients from fungal partners and represent poorly understood components of forest biodiversity. Many are rare or endangered yet can be ecological indicators of forest ecosystem function due to their often highly specific fungal host requirements. One such species is the IUCN red-listed (vulnerable), fully mycoheterotrophic orchid, Corallorhiza bentleyi. This recently described species is among the rarest plants in Appalachia, known from five counties in Virginia and West Virginia, USA. The species has a restricted range, small population size, and is self-pollinating. Here we take an integrative approach to conservation genetic assessment in C. bentleyi using floral morphometrics, simple-sequence repeats, and fungal host DNA to characterize variation within and among sampling localities. Morphology reveals some differentiation among individuals from six sampling localities. Surprisingly, most genetic variation is found within localities, contra to the expectation for a selfing species. Fungal host DNA reveals extreme specificity upon a few genotypes of a single ectomycorrhizal host species, Tomentella fuscocinerea, across all localities. We discuss the conservation implications of morphological, genetic, and symbiotic diversity in this vulnerable species, and recommend additional assessment of conservation status based on: an obligate reproductive mode of selfing, preventing benefits of outcrossing among genetically non-identical individuals; extreme host specificity, severely restricting niche space; and highly fragmented habitat under threat from anthropogenic disturbance. This study underscores the importance of integrative conservation assessment, analyzing multiple data sources, and reveals patterns not readily apparent from census-based assessments alone.

ISSRseq: An extensible method for reduced representation sequencing

“…Additional evidence of the recovery of population genomic signal in these data was our identification of fixed homozygous sites exclusive to cleistogamous individuals. We find the general congruence of visually scored ISSR banding patterns in Fama et al (2021) with the more sensitive, sequence-based nature of ISSRseq to be additional corroboration of our new method.…”

“…ISSRseq corroborated results of a traditional ISSR investigation study of C. bentleyi conducted previously by our group. Fama et al (2021) scored ISSR bands visually for two primers used in this study and found that 89% of molecular variance occurred within populations.…”

“…We selected C. bentleyi to determine whether our method was informative below the level of species, C. striata to evaluate the performance across putative species boundaries, and P. deltoides to test ISSRseq within a single accession of a species with a well-characterized genome. We collected 87 individuals from 28 C. striata localities, as in Fama et al (2021), Barrett and Freudenstein (2011) and Barrett et al (2018; Table 1), and 37 individuals from six C. bentleyi localities. Samples of C. striata were collected from 10 US states and two Canadian provinces (British Columbia and Manitoba, Table 1).…”

Weak population spatial genetic structure and low infraspecific specificity for fungal partners in the rare mycoheterotrophic orchid Epipogium aphyllum

“…Nevertheless, in mycoheterotrohic orchids, cryptic specialisation for fungi may arise despite high gene flow if plants' fungal preferences are selected in a local environment throughout the species range, as expected in the geographic, mosaic model of coevolution (Thompson 2005). Unfortunately, population genetic studies that could help to elucidate the extent of genetic variation as well as patterns of gene flow for mycoheterotrophic plants (orchids in particular) are still rare (but see Alves et al 2021;Beatty and Provan 2011a, b;Fama et al 2021;Hopkins and Taylor 2011;Klooster and Culley 2010).…”