Adjoa Richardson Ahedor, Jennifer Messick, Wayne J. Elisens, Abigail J. Moore

Genetic Structure of the Mecardonia acuminata (Plantaginaceae) Complex in the Southeastern USA

Introduction

Genetic structure of the mecardonia acuminata (plantaginaceae) complex in the southeastern usa. Examines the genetic structure of Mecardonia acuminata in the southeastern USA using ISSR data. Reveals genetic groups inconsistent with morphology/geography, suggesting ongoing diversification.

Abstract

Mecardonia acuminata (Plantaginaceae) is found in the southeastern United States and has traditionally been divided into three varieties. A quantitative analysis of morphological data supported the division into the three varieties, although the ranges of the varieties found in that study were not the same as their traditional ranges. Here we use ISSR data to examine the relationships of 238 individuals from 23 locations throughout the range of M. acuminata. Although there is genetic structure that is congruent across different analyses, the groups recovered are not consistent with morphology or geography. The results indicate eastern-western distributions of the species with centers of diversity not only in the south but as far north as the Central Basin of the Interior Low Plateaus. The results further suggest ongoing diversification of lineages of M. acuminata, or the presence of widespread genes that govern the morphological traits that are traditionally used in delimitating the varieties.

Review

This manuscript presents a timely and valuable investigation into the genetic structure of the *Mecardonia acuminata* complex, a species traditionally divided into three varieties within the southeastern USA. Building upon previous morphological analyses that supported varietal distinctions but challenged traditional range delimitations, this study employs ISSR markers to provide a genetic perspective on the relationships within this complex. Understanding the genetic architecture of such widespread species is crucial for clarifying taxonomic boundaries, elucidating evolutionary processes, and informing conservation strategies, making this a significant contribution to plant systematics. The authors utilized ISSR data from a robust sampling of 238 individuals across 23 locations, providing a comprehensive genetic overview of *M. acuminata*. A key strength of the study is the identification of clear and congruent genetic structure, which is critical for supporting the validity of the findings. However, the most compelling and indeed provocative finding is the lack of congruence between these genetic groups and both traditional morphological varieties and their geographical distributions. Instead, the study reveals an east-west distribution pattern with surprisingly widespread centers of diversity, extending as far north as the Central Basin of the Interior Low Plateaus. The authors offer two insightful interpretations for this discordance: either ongoing diversification processes within the complex, or the presence of widespread genes underlying the morphological traits used for traditional varietal delineation. The results profoundly challenge the current taxonomic understanding of *M. acuminata*, suggesting that morphological traits may not accurately reflect underlying genetic divergence in this system. This calls for a re-evaluation of the varietal distinctions and highlights the complex interplay between morphological plasticity and genetic differentiation. Future research could benefit from integrating additional molecular markers (e.g., SNPs, sequence data) to provide finer resolution and further test the hypotheses of ongoing diversification or widespread morphology-governing genes. Overall, this study presents a well-executed and thought-provoking analysis that significantly advances our understanding of *M. acuminata*, laying excellent groundwork for future research into its evolutionary history and taxonomy.

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