DIPLOID‐TETRAPLOID SYMPATRY IN DACTYLIS (GRAMINEAE)

S ummary The known zones of contact between diploid and tetraploid Dactylis are reviewed, and two well‐documented examples of intermixed and morphologically similar diploids and tetraploids are considered in detail. The relict subspecies mairei is accompanied, in its particular habitat, by a tetraploid counterpart. The evidence suggests this to be an example of autotetraploidy, the diploid and tetraploid components of which have been preserved by isolation in a specialized relict habitat. In Dactylis interploid sympatry appears to have arisen in two ways–by changes in the original habitat, whereby morphologically dissimilar and spatially isolated diploids and tetraploids become intermixed, or by the formation of autotetraploids which co‐exist in the‘dipioid’habitat and may subsequently be modified by hybridization with other diploids or tetraploids with which they come in contact. No triploid or tetraploid hybrids were discovered in the seedling progeny of dipioid Kerrata plants, hut the numbers screened are considered insufficient. Direct formation of tetraploid hybrids as a consequence of the functioning of unreduced gametes by dipioid parents can be a more effective‘genetic bridge’than the backcrossing of triploid hybrids. Further study of mixed diploid‐tetraploid stands is therefore required to determine the frequency and effectiveness of this phenomenon in natural populations.