Confirmation of the Hybrid Origin of Chiloglottis × pescottiana (Orchidaceae: Diurideae). I. Genetic and Morphometric Evidence

Abstract

Pollination by sexual deception in orchids is characterised by a high degree of pollinator specificity, which may account for the rarity of natural hybrids within the group. Only one such hybrid has been formally recognised in Australia, Chiloglottis × pescottiana R.S.Rogers, which has intermediate floral morphology between its putative parents, C. valida D.L.Jones and C. trapeziformis Fitzg. In this paper, genetic and morphometric analyses confirm the hybrid origin of this taxon. Allozyme analysis of C. trapeziformis and C. valida revealed fixed allelic differences at four ‘diagnostic’ loci and significant frequency differences at three other loci. In all cases, C. × pescottiana exhibited fixed heterozygosity at the diagnostic loci. Multidimensional scaling of both the genetic data and seven morphometric traits revealed distinct clusters of C. trapeziformis and C. valida while C. × pescottiana formed an intermediate cluster between the two parents. To test for genetic compatibility between C. trapeziformis, C. valida and C. × pescottiana, a series of reciprocal artificial crosses were performed. In all cases, the percentage of capsules formed was at least as great for between-species crosses as for within-species selfs and crosses (range 75–100%). No significant differences in the percentage of seed with normal embryos was detected between self- and cross-pollinations within C. trapeziformis (range 77–81%), C. valida (range 59–74%) and C. × pescottiana (range 30–51%), but the percentage of normal embryos was notably lower in C. × pescottiana. The cross C. trapeziformis female by C. valida male produced significantly more normal embryos (90%) compared with the reverse cross (46%). Artificial backcrosses of C. × pescottiana to C. valida and C. trapeziformis had lower percentages of normal embryos when C. × pescottiana was the pollen donor (39–43%) rather than recipient (62–68%), suggesting reduced pollen viability in the latter taxon. The size of F2 embryos in C. × pescottiana seed capsules was smaller than the embryos of both C. valida and C. trapeziformis. Despite confirmation of hybridisation, little evidence for backcrossing was found. Thus, while the specific pollinator relationships may occasionally break down in these sexually deceptive orchids, reduced viability of hybrid pollen and F₂ seed, and inefficient pollination of the hybrid, may minimise introgression. It is concluded from the available evidence that hybridisation has not been a major evolutionary factor in the diversification of sexual deception worldwide.