Observations of high genetic variability in the endangered Australian terrestrial orchid Pterostylis gibbosa R. Br. (Orchidaceae)

The genetic variation in all known populations of an endangered Australian native terrestrial orchid Pterostylis gibbosa R.Br., was investigated with starch gel electrophoresis. A total of 16 isozyme loci were assayed. The percentage of polymorphic loci (P), the number of alleles per locus (A), observed and expected heterozygosity at population levels were 69%, 2.21, 0.210, 0.261, respectively. The G(st) value of 15% indicates that around 85% of variation resides within populations. Despite isolation by distance most alleles were distributed across most of the populations. High genetic variability along with low population divergence may be the result of recent population fragmentation or from extensive gene flow maintained by seed and pollen movement. To investigate whether poor seed viability contributed towards its rarity, an orchid seed viability test using Fluorescein diacetate revealed high seed viability (range 68-90%). Although endangered and restricted to only four geographical areas, P. gibbosa showed a higher level of genetic variation than other orchids with larger populations.