Cytogenetic studies on microsporogenesis and male gametophyte development in autotriploid cucumber (Cucumis sativus L.): implication for fertility and production of trisomics

Abstract

To understand the correlation between chromosomes behavior and fertility in autotriploid cucumber (Cucumis sativus L.), microsporogenesis in pollen mother cells (PMCs) and male gametophyte development were studied using improved staining and chromosome preparation techniques. Meanwhile, for more efficient selection of trisomics from the progeny of autotriploid-diploid crosses, fertilization rates of ovules from reciprocal crosses were counted to observe the transfer rate of gametes in the autotriploid cucumber. Variable chromosome configurations, e.g. multivalents, quadrivalents, trivalents, bivalents and univalents were observed in the most PMCs of the autotriploids at metaphase I. Chromosome lagging and bridges at anaphase in both meiotic divisions resulted from irregular chromosome separation and asynchronization was frequently observed as well, which led to formation of micronuclei and inviable gametes. The frequency of normal PMCs in autotriploids at the stage of tetrad was only 40.6%. Among those normal microspores, most of them (91.2%) could develop into normal gametophytes with 2 cells and 3 germ pores. Stainability and germination rate of pollen grains were only 18.8 and 13.5%, respectively. However, chromosomes separated to form gametes with 8 chromosomes at anaphase I, suggesting a possible method for the production of primary trisomics from the progeny of autotriploid-diploid crosses. Fruit set of 3n × 2n and 2n × 3n were 80 and 70%, respectively. It obtained an average of 6.2 plump seeds per fruit in 3n × 2n, while 4.9 in 2n × 3n crosses. Transfer rates of gametes through the gastrula or the pollen in autotriploids were 13.4 and 10.4%, respectively. Some aneuploid gametes (n + 1 = 8, n + 2 = 9) also have capability of setting seed and sexual reproduction besides normal gametes containing whole chromosome sets (n = 7, 2n = 14). Further, some primary trisomic plants were selected from the progeny of autotriploid-diploid crosses. Based on the results obtained we suggest that abnormal meiosis in PMCs was the cytogenetic reason for low fertility of autotriploid cucumber pollen. 3n × 2n cross was more efficient for selecting primary trisomic plants in cucumber.