A genetic comparison of bog vs. forest populations of Metrosideros polymorpha (Myrtaceae) in Hawai'i

Metrosideros polymorpha is the most abundant native plant in the Hawaiian Islands growing at elevations from sea level to the subalpine. M. polymorpha exhibits high levels of apparent local adaptation and ranges in morphology from small shrubs (<1m) to relatively large trees (>20m). Despite the high morphological variation and broad ecological amplitude in this species, there have been few studies assessing genetic variation among populations of morphological varieties. The objective of this study was to use the molecular technique of inter-simple sequence repeats (ISSRs) to examine the genetic diversity and structure of morphologically distinct neighboring populations of M. polymorpha, growing in bog or bog-like conditions and adjacent or nearby forests across multiple islands. ISSR data using three primers were collected for a total of 287 individuals from five of the major islands. A total of 111 loci were found to be 100% polymorphic. The mean value of Nei's gene diversity for all populations was 0.2436 +/- 0.172. The majority of genetic variation was found within microhabitat within islands, with an average of 91.34% (range 80.87%--95.72%). The average amount of genetic variation attributed to differences among microhabitats across islands was 8.64% (range 4.28%--19.13%). There was a significant correlation between geographic and genetic distance across all populations, and a UPGMA phenogram shows the Kaua'i bog population to have the greatest genetic distance from all other populations. This study demonstrates that populations of morphologically distinct variants of M. polymorpha contain an average amount of genetic diversity within populations and a low amount of genetic differentiation among populations compared to other flowering plant species. These data reflect the fact that M. polymorpha is a widespread ecological generalist capable of living in a vast range of habitats most likely due to extensive gene flow throughout the Hawaiian Islands. Detectable levels of genetic differentiation among populations appear to be the result of geographic isolation rather than putative adaptation to microhabitats, and therefore the different morphologies of bog vs. forest plants are most likely due to phenotypic plasticity and may not have a strong genetic basis.