Conservation genetics of relict tropical species of Magnolia (section Macrophylla)

Angélica Chávez-Cortázar,Maried Ochoa-Zavala,Ken Oyama,Marie-Stéphanie Samain,Emily Veltjen,Mauricio Quesada,Martín Mata-Rosas

doi:10.1007/s10592-021-01334-5

Abstract

Special conservation efforts should be made for relict species, as they usually have small population sizes and restricted distributions, placing them in critical extinction risk. To achieve conservation, information about genetic diversity distribution is needed. Here, using nine nuclear microsatellites, we analyzed 23 populations of five recently described species of Magnolia distributed in Mexico, which were previously assigned to Magnolia dealbata. We aimed to determine the level of genetic diversity and the distribution of genetic variation and proposed conservation measures. Compared to other endemic and relict species, we found a moderate level of genetic diversity in most populations; however, we identified two populations with no genetic variation. Additionally, we found evidence of positive values of inbreeding likely due to geitonogamy. We found a strong population structure, low effective population size, and no evidence of bottlenecks. Patterns of genetic differentiation did not support the morphological distinction of five species, so we hypothesized that the gene pools may instead represent well-differentiated populations of a single species. We argue that the pattern of genetic differentiation is explained by the natural fragmentation of the cloud forests after glaciation events, and the effects of genetic drift in small populations poorly connected by gene flow. Despite the moderate levels of genetic diversity, special attention is needed to guarantee conservation, with emphasis on the populations in the central region of the country as well as the valuable populations identified in the southwestern region.

Highlights

The fact that relict species usually represent descendants of once widespread taxa suggests that current populations are small in size and occupy disjunct and restricted distributions, placing them in an elevated extinction risk (Habel et al 2010)
Because limited information is available regarding the genetic diversity, and distribution of genetic variation in Magnolia in Mexico, this study focused on five of the seven species conforming to the Macrophylla section in Mexico: M. nuevoleonensis, M. alejandrae, M. rzedowskiana, M. vovidesii, and M. dealbata
The morphological characters can fail to differentiate between cryptic species or incorrectly subdivide species because of a misinterpretation of natural morphological variation across their distribution range (Duminil and Di Michele 2009; Duminil et al 2012)

Summary

Introduction

The fact that relict species usually represent descendants of once widespread taxa (or populations) suggests that current populations are small in size and occupy disjunct and restricted distributions, placing them in an elevated extinction risk (Habel et al 2010). The intriguing disjunct distribution of plants in the Northern Hemisphere have been of great interest for evolutionary studies Such disjunction is thought to be relict of a more continuously distributed mesophytic forest during the Cenozoic (66 Ma), known as the Boreotropical flora, which became fragmented due to geologic and climatic changes (Azuma et al 2001; Nie et al 2008). One of the former Boreotropical element is the family Magnoliaceae, which subfamily divergences date back to the Cretaceous Members of the genus produce secondary metabolites (e.g. alkaloids and flavonoids), which appear to be involved in defense against natural enemies playing an important role in the evolution of Magnoliaceae (Thien 1996)

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