Apomixis, hybridisation, polyploidy and crossability in the genus Fragaria
The relationship of parental genetic distance with cross success and progeny fitness was investigated in crosses made within diploid Fragaria. The mean values for all three components of success rate of crosses (measured as berry set, achene set and seed germination) at intra- and interspecific levels were not correlated with parental genetic distance. While mean berry set and seed germination at cross types was not affected by parental genetic distance, mean achene set was higher in crosses with intermediate genetic distance and lower in crosses between both more closely and more distantly related taxa, suggesting both inbreeding and outbreeding depression effect on achene set. Three components of progeny fitness (assessed as progeny flowering frequency, berry set and pollen sterility) were strongly correlated with parental genetic distance in both intra- and interspecific crosses. By increasing parental genetic distance, flowering frequency and berry set consistently decreased while pollen sterility increased. Therefore, outbreeding depression has strongly affected all three components of progeny fitness. Marked outbreeding depression was evident on flowering frequency and berry set at very short genetic distances between subspecies, and even within subspecies at forma levels, while it was not evident in pollen sterility of progeny at short genetic distance within subspecies. Highly male sterile plants were found among progeny from crosses within F. vesca ssp. Vesca. As the plants had high levels of berry set, the occurrence of high male sterility among progeny from more closely related taxa could be interpreted as a result of hermaphroditism suppression and evidence of a trend towards gynodioecy in this taxon.