Triploid flowering pears reduce self-sowing
Flowering pear trees are popular additions to landscapes across much of the United States. The attractive ornamentals are favored for their abundance of white flowers, showy fall color, broad pest resistance, striking forms, and ability to thrive in the USDA plant hardiness zones 5 to 8. However, in some areas, flowering pears reseed and naturalize prolifically, creating issues that have caused one variety to be listed by the U.S. Fish and Wildlife Service as a "plant invader" in mid-Atlantic natural areas. "Birds eat the fruits and disperse seeds into nearby areas where it commonly naturalizes in old fields and along highways," explained Thomas Ranney, corresponding author of a study in the August 2016 issue of HortScience. Ranney and coauthors Whitney Phillips, Darren Touchell, and Thomas Eaker studied flowering pear hybrids to investigate options for developing infertile cultivars.
"Development of highly infertile cultivars would be desirable as an alternative to the fertile cultivars currently available," the authors said. "One approach for producing seedless plants is through the development of triploids." Triploids typically are highly infertile due to a reproductive barrier that has three sets of chromosomes. The sets of chromosomes cannot be divided evenly during meiosis, yielding unbalanced segregation of chromosomes. Breeding of triploid plants has been used successfully to develop seedless cultivars of crops such as bananas, watermelons, and some citrus.
The study involved triploid Pyrus hybrids established in a field at the Mountain Horticultural Crops Research and Extension Center in Mills River, North Carolina. Thirteen triploids were selected from the original population based on desirable traits, including heavy flowering, desirable forms, absence of fire blight, and attractive fall color.
The researchers determined female fertility of each triploid by comparing the number of seedlings per flower of a triploid to the most fertile diploid control. Fertility was characterized by evaluating fruit set, seeds per fruit, seed germination, seedlings per flower, and percent relative fertility. Data showed that many triploids displayed a substantial reduction in fertility (as much as 100%). Relative female fertility varied considerably among accessions and ranged from 0.0% to 33.6%. Of the 13 triploids used in this study, five accessions had a relative fertility of <2%.
Flow cytometry was used to determine relative genome sizes and ploidy levels of female parents, seedlings, and seeds (both embryo and endosperm) and to make inferences regarding reproductive pathways. "Analysis of seeds and seedlings from triploid maternal parents showed that they were predominantly abnormal aneuploids, which typically results in seedlings with reduced fitness and fertility," the authors said.
"These results indicate that selection of highly infertile triploid cultivars is a viable approach to reduce or eliminate the self-sowing of flowering pears in the landscape," the authors concluded.