2019 ASHS Annual Conference

Althea (Hibiscus syriacus) is a hardy shrub valued for its showy summer flowers and adaptability. Flowers include an array of colors and also may be single, semi-double, or double. Its naturally occurring cytotype is tetraploid (2n = 4x = 80) but breeders have developed higher ploidy plants to reduce fertility. Both flower form and ploidy have been shown to impact fertility, but quantification remains incomplete. The objectives of this study were to 1) quantify female fertility of 4x, 5x, and 6x genotypes and 2) quantify female fertility of single, semi-double, and double flowers. Each taxon from these representative groups were pollinated with a variety of male-fertile genotypes. Single and semi-double flowers did not differ in female fertility (9.7 and 10.0 seeds per pollination) but double flowers had only 2.2 seeds per pollination. Pollinating tetraploids yielded an average of eight seeds per pollination and ranged from zero to 32 seeds per pollination. Taxa with genome sizes in the range of pentaploid (5.7 to 6.8 pg) had an average of three seeds per pollination. However, there were four outliers that stood out in the data and after removing these accessions there was 0.9 seeds per pollination. The lowest fertility was observed among the three cytotypes was for hexaploid taxa, with only 0.05 seeds per pollination after making 402 pollinations. It is unclear why the pentaploid taxa were more female fertile compared to hexaploids in this study, but several points are worth noting. First, there was a high degree of variability in female fertility among tetraploid taxa that were expected to be fully fertile, suggesting greater complexity in the regulation of fertility – possibly involving inbreeding depression during cultivar development. Second, hexaploids plants likely produce more multivalents that result in improper disjunction during gamete formation. Relatedly, the so-called pentaploids may actually be aneuploids that lose extra chromosomes during gamete formation and result in euploid gametes (e.g. n = 2x = 40) during egg formation. Finally, using hexaploids as female in crosses with tetraploid males represents further deviation from standard maternal:paternal contribution (2:1). Specifically, the theoretical maternal: paternal contribution of 6x x 4x crosses is 3:1, while that of 5x x 4x crosses is 2.5:1. To draw conclusions genome size analysis of all interploid progeny as well as cytological analysis of 5x and 6x plants is necessary.