MEIOTIC RESTITUTION AND THE ORIGIN OF POLYPLOIDY. III. THE CYTOLOGY AND FERTILITY OF EIGHT GENERATIONS OF THE OFFSPRING OF A SPONTANEOUSLY PRODUCED AMPHIPOLYPLOID OF TRITICUM CRASSUM × T. TURGIDUM

Abstract

The pentaploid F1 hybrid (2n = 35) between Triticum crassum (2n = 42 = DDD2D2McrMcr) and T. turgidum (2n = 28 = AABB) produced spontaneous, polyploid F2 offspring with 2n = 70 chromosomes (AABBDDD2D2McrMcr). These offspring were maintained for eight successive generations. From each generation the most fertile plants were selected to serve as parent plants for the next generation.Cytological studies showed that 14 chromosomes were rapidly eliminated in the early generations; the chromosome number became stabilized at 2n = 56 in the F5. The chromosome behavior was very irregular in the meioses of the early generations, but gradually became more regular in the later generations. Pollen mother cells with chromosome mosaics, a very dominant feature of meiosis the F2 and F3 generations, reduced to an occurrence of about 10% in the F7, F8 and F9 generations. The number of univalents decreased while the chiasma frequencies per cell increased. Simultaneous with the improvement of the meiotic behavior of the chromosomes, the fertility increased from an average of less than 10% in the F2 to about 50% in the F7, F8 and F9 generations. The maximum fertility of individual plants was between 80 to 90%. There was thus a clear relationship between the observed pattern in fertility and meiotic chromosome behavior in response to selection for high fertility. This relationship is discussed in detail. The offspring has all characteristics of becoming a separate, polyploid species.