The phylogenetic position of the morphologically unusual Pleurostichidium falkenbergii (Rhodomelaceae, Rhodophyta) based on plastid phylogenomics

Pleurostichidium falkenbergii is an obligate epiphyte on Xiphophora chondrophylla and is endemic to northern New Zealand. This monotypic genus is characterised by having dorsiventral and laterally compressed thalli with 20 pericentral cells and complete cortication, adventitious trichoblasts, spherical spermatangial branches formed from cortical cells, and stichidia with up to 19 whorled tetrasporangia. This unusual combination of morphological characters among the Rhodomelaceae led to its placement in a separate tribe Pleurostichidieae. The tribal phylogenetic placement has been investigated only using 18S rRNA and remains uncertain. We investigated the phylogenetic affinities of P. falkenbergii using a plastid phylogenomic approach. We sequenced and assembled its plastid genome and inferred a phylogeny along with 45 other rhodomelacean species and 10 outgroup taxa. P. falkenbergii was placed in a highly supported clade containing the tribes Pterosiphonieae, Dipterosiphonieae, Herposiphonieae, and a clade with two species morphologically assigned to Polysiphonia and Melanothamnus, but genetically unrelated to the true Polysiphonia and Melanothamnus lineages. Pleurostichidium falkenbergii was placed as sister to the Pterosiphonieae with moderate support. Considering our results and the unusual combination of vegetative and reproductive characters of Pleurostichidium, we propose to maintain the tribe Pleurostichidieae for this species.