Aneuploid strawberry (2 n = 8 x + 2 = 58) was developed from homozygous unreduced gamete (8 x) produced by second division restitution in pollen

Abstract

Unreduced gamete formation is significant in the evolutionary development of complex polyploidy series found in wild strawberry, genus Fragaria (Rosaceae). Also, it is important for genetics and breeding in strawberry plants to elucidate the mechanism of unreduced gamete formation. The objective of this study was to search for ploidy anomalies resulting from artificial diploid × octoploid crosses, and examine the mechanism through which these unreduced gametes were produced. Five everbearing cultivars of Fragaria vesca L. diploid (2 n = 2x = 14) were crossed with pollen from six June-bearing cultivars of Fragaria × ananassa Duch., octoploid (2 n = 8x = 56). A total of 3000 mature seeds, 100 from each of the 30 parental combinations were sown at 23 °C/20 °C (day/night) under artificial lighting with a 16 h day. The seedlings were transplanted to pots and grown in a greenhouse. Reproductive and morphological observations, flow cytometry analyses, chromosome counts and DNA analyses using CAPS markers were performed to identify the genetic background of the offspring. Most of the seed (79%) did not germinate or died soon after germination. Of the seedlings produced, 7% seemed to be pure F. vesca based on morphological characteristics, flow cytometry analyses and chromosome counts; 14% were pentaploids (2 n = 5 x = 35), 0.1% were hexaploids (2 n = 6 x = 42), and 0.03% (one individual) was aneuploid (2 n = 8 x + 2 = 58). Electrophoresis banding patterns obtained by CAPS marker analysis were heterozygotic in the 8 x pollen parent but homozygotic in the aneuploid progeny. Judging from the chromosome counts and the CAPS marker analysis, the aneuploid was the result of a homozygous unreduced pollen grain (8 x) crossed with an incomplete chromosome compliment from the egg. Because of the homozygosity, the unreduced male gamete must have been derived from second division restitution (SDR) in the octoploid pollen parent.