The Sticta filix - Sticta lacera conundrum (lichenized Ascomycota: Peltigeraceae subfamily Lobarioideae): unresolved lineage sorting or developmental switch?

Abstract

Abstract We assessed the status of two New Zealand endemic morphodemes in the genus Sticta, currently treated as two separate taxa, Sticta filix and Sticta lacera. Both are green-algal lichens with a distinct stipe that grow in forested habitats and are suitable indicators of the indigenous vegetation health in forest ecosystems in New Zealand. They exhibit different morphologies and substrate ecologies: S. filix forms rather robust thalli, often on exposed trunks of phorophytes, with erect stems distinctly emerging from the substrate, whereas S. lacera is a more delicate lichen growing near the base of trees, usually among bryophyte mats or sheltered in the exposed portions of the phorophyte root-plate, with a prostrate, branched, stolon-like stem barely emerging from the substrate. Throughout their range, both taxa grow sympatrically and often in close proximity (syntopically). Despite the differences, ITS barcoding does not support the two morphodemes as separate species. In this study we assessed two possible explanations: (1) S. filix and S. lacera are discrete phenotypes of a single species, caused by developmental switching triggered by a discrete environmental variable, the propagules developing either on bare substrate or between bryophytes; and (2) the two morphodemes represent separate lineages, but ITS does not provide sufficient resolution to reflect this. We performed a quantitative analysis of morphological and ecological parameters, based on vouchered herbarium collections and field observations on iNaturalist NZ (https://inaturalist.nz), to assess the level of discreteness of the growth forms and to test for a correlation with the presence of a bryophyte mat. We further took advantage of an existing molecular data set from a target capture approach, comprised of 205 protein markers. This data set was used to establish a framework of percentage identities between pairs of the same and of different species among lobarioid Peltigeraceae and then to test whether the S. filix/lacera pairing fell closer to a within-species or a between-species pairing. The morphometric analysis of herbarium material resolved S. filix and S. lacera as two discrete morphs with little overlap, supported by numerous observations on iNaturalist NZ. However, whereas herbarium material suggested a significant association of the lacera morph with bryophyte mats, no such pattern was evident from field images on iNaturalist NZ, in which both morphs frequently associated with bryophyte mats. This highlights the limitations of herbarium material to correctly assess substrate ecology, whereas iNaturalist NZ postings had issues with correct identifications, given that especially S. lacera is easily confused with Pseudocyphellaria multifida. Based on the target capture data, the percentage identity of the S. filix/lacera pairing (99.43%) was significantly higher than that of all 12 between-species pairings (93.20–98.01%); it was at the same time lower than that of all within-species pairings (99.63–99.99%) but significantly so only in comparison with five out of the eight within-species pairings. The target capture approach is thus inconclusive, but the combination of all data suggests that S. filix and S. lacera are not discrete morphodemes of a single species but represent two separate lineages which emerged recently and hence cannot be resolved using the ITS barcoding marker or even a deeper phylogenomic approach based on protein-coding markers. We propose transplantation experiments and the application of RADseq to further assess this situation.