Artificial Hybridization in Hedeoma (Labiatae)

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Abstract
Thirteen species of Hedeoma [H. drummondii, H. reverchonii var. reverchonii, H. reverchonii var. serpyllifolium, H. diffusum, H. hyssopifolium, H. pulcherrimum, H. nanum var. macrocalyx, H. oblongifolium, H. plicatum, H. multiflorum, H. montanum, H. palmeri and H. reverchonii] representing 3 subgenera and Poliomintha longiflora were artificially crossed to attempt 52 hybrid combinations. Crossability was generally high and 83% of the attempted hybridizations between species pairs were successful. Intrasubgeneric hybrids within the subgenus Saturejoides were the most easily obtained, whereas intersubgeneric and intergeneric hybridization attempts accounted for most of the crossing failures. Only 9 F1 hybrids set viable seed, and almost all of these were intrasubgeneric crosses within subg. Saturejoides. Hybrid sterility is seemingly an important reproductive barrier in Hedeoma; translocations and inversions were among the observed meiotic irregularities. One nearly sterile hybrid was selfed to produce a relatively fertile F2 generation. Intercorrelation analysis of the F2 revealed significant correlations only among related morphological characters and no significant correlations were found between morphology and pollen stainability. At least in this instance, the barrier of hybrid sterility can apparently be easily overcome and hybridization may be a source of naturally occurring interpopulational variation.