Striking genetic homogeneity in the widespread South American Bracken

Abstract

Premise: Bracken (Pteridium, Dennstaedtiaceae) is a cosmopolitan genus of aggressive 39 disturbance colonizers that are toxic to agricultural livestock. The taxonomy of Pteridium has 40 been treated in multiple schemes, ranging from one to six species worldwide, with numerous 41 subspecies and varieties. Recent work has focused on the worldwide distribution and 42 systematics of the bracken fern, but South America has been poorly represented.Methods: Here, we present the first continent-wide sampling and analysis of Pteridium 44 esculentum, a southern hemisphere diploid species. Within South America, P. esculentum has 45 several morphotypes, distinguished into subspecies by variation in indument and lamina 46 architecture. In this study, we use double digest restriction site-associated DNA sequencing 47 (ddRADSeq) to assess the phylogenetic relationships of P. esculentum subspecies. 48 Results: We found a striking genetic homogeneity for the species, being able to support only 49 two morphotypes from molecular data: P. e. arachnoideum and P. e. campestre. We had high 50 confidence for shallow and deep phylogenetic relationships, but less support for relationships 51 among crown groups. 52 Conclusions: We describe an east-west geographic pattern that would explain the 53 relationships between populations and, in contrast to previous studies, we detected 54 differences with P. esculentum from Australia. These results will lay the foundations for 55 studying variations in its behavior as a weed, as well as its impact on the production of 56 agricultural livestock in South America.