Variation in flowering phenology and selfing rate across a contact zone between diploid and tetraploid Arrhenatherum elatius (Poaceae)

Plant populations with different ploidy levels have frequently been found to develop mechanisms of reproductive isolation when they occur in contact, presumably because of selection against the production of inviable or sterile hybrids. A parapatric contact zone between diploid and tetraploid Arrhenatherum elatius has been identified in the Spanish Pyrenees. Of 145 flowering plants sampled in the zone one was triploid. This study provides data on enzymatic allele frequencies, flowering time and selfing rate across the contact zone, to examine whether reproductive isolation occurs to limit the potential production of triploids. To substantiate these data, flowering time was measured in plants grown in a common garden and outcrossing rates were compared to those in one allopatric population of each cytotype from an environmentally similar habitat. Nei's genetic distances calculated on the basis of allele frequencies showed no clear evidence for past hybridization and/or introgression between diploids and tetraploids. Patterns of flowering phenoloy in the field and in controlled conditions indicated almost total flowering divergence between parapatric cytotypes; this divergence may be under genetic control. There was, however, no evidence of current phenotypic selection on flowering date in the contact zone. For each cytotype, multilocus selfing rates were higher in parapatric populations than in allopatric populations. The possibility that flowering time divergence and increased selfing have evolved in the contact zone resulting in reproductive isolation is discussed.