Maintenance of high levels of genetic diversity and significant fine-scale genetic structure among age classes in single-tree populations of the endangered epiphytic orchid Laelia speciosa (Orchidaceae) in central Mexico

Abstract

<p><strong>Background: </strong>Epiphytic plants exist within the complex three-dimensional habitat of the tree canopy. This can have a profound effect on patterns of both genetic diversity and structure and may be important for conservation of epiphytic species.<strong> </strong></p><p><strong>Objective:</strong> To evaluate how the epiphytic habit affects fine-scale three-dimensional genetic structure in single canopy tree populations.  </p><p><strong>Studied species:</strong><em> Laelia speciosa</em>, a threatened endemic Mexican epiphytic orchid.<em>         </em></p><p><strong>Study site and dates:</strong> Field sampling occurred during Spring 2006 in the municipality of Caurio de Guadalupe, Michoacán.</p><p><strong>Methods: </strong>All individuals occurring in two trees of <em>Q. deserticola</em> (178 in Tree 1 and 276 in Tree 2) were mapped, assigned to four discrete age classes, and genotyped at six microsatellite loci to understand patterns of genetic diversity and structure. </p><p><strong>Results:</strong> High levels of genetic diversity (polymorphic loci = 100 %, <em>Ho</em> = 0.398 – 0.506) occurred across size classes and trees but were not significantly different between the two trees. Significant fine-scale genetic structure was found in the Juvenile (plants 2-10 years old) and Adults I (11-20 years old) age classes in distances up to 15 cm. This significant structure was larger (up to 25 cm) when all individuals were treated as a single age class.<strong></strong></p><strong>Conclusion: </strong>The distribution and structure of genetic diversity is attributable to the epiphytic habit, particularly localized seed dispersal, limited selfing, and selection of adaptive traits.  Conservation implications related to the sampling for <em>in vitro</em> propagation are given in light of the metapopulation genetic variability maintained within single-tree populations.