Cell development in the anther, the ovule, and the young seed of <i>Triticum aestivum</i> L. var. Chinese Spring

Abstract The developmental behaviour of reproductive cells was studied during premeiotic mitotic activity, premeiotic interphase, meiosis in anthers and ovaries, microsporogenesis, megasporogenesis and embryo and endosperm growth in Triticum aestivum L. var. Chinese Spring. Particular attention was paid first to the timing and rate of cell development in the anthers and the ovary within a floret and secondly, to the timing and rate of nuclear and cell development in the young embryo and endosperm. At 20 °C the development studied lasted in each floret about 21 days starting 7 days prior to meiosis in anthers and ending 5 days after anther dehiscence and pollination. The durations of up to twenty successive cell cycles were estimated. In anthers of plants grown at 20 °C the durations of the three successive cell cycles immediately prior to the cycle which ends at first anaphase of meiosis were about 25, 35 and 55 h respectively. The increase in cell cycle time was correlated with an increase in the size of archesporial cells and their nuclei. A progressive increase in the durations of successive cell cycles as meiosis is approached has not been measured previously in a higher plant species, although it has been noted in the germ line cells of male mice. The pollen mother cells (p.m.cs) within an anther were synchronized prior to meiosis by having their development blocked somewhere in G1 of premeiotic interphase. The developmental hold began to operate about 103 h prior to the synchronous onset of meiosis in all the p.m.cs within an anther at 20 °C. About 55 h later, when the last archesporial cell completed its final premeiotic mitosis, all the p.m.cs had accumulated in G1 of premeiotic interphase and synchrony was complete. Premeiotic interphase after all the p.m.cs were first synchronized in G1 until the synchronous onset of meiosis lasted about 48 h. During this period the G1 developmental hold was released and p.m.cs initiated DNA synthesis synchronously about 12 to 15 h prior to the start of leptotene. Meiosis in p.m.cs lasted 24 h at 20 °C. Within each floret, meiosis in p.m.cs was almost or quite synchronous with, and had the same duration as, meiosis in the embryo sac mother cell. The Q10 for meiosis in p.m.cs over the temperature range 15 to 25 °C was about 2.3. Microsporogenesis from tetrad stage until anther dehiscence lasted about 7.5 days at 20 °C. The first pollen grain mitosis (p.g.m. 1) occurred 2.5 days and second pollen grain mitosis (p.g.m. 2) 5.0 days after the end of meiosis. Concurrent with p.g.m. 1 the functional megaspore in the ovule of the same floret divided. This division was rapidly followed by two more synchronous division cycles (also concurrent with p.g.m. 1) which produced an 8-nucleate embryo sac. By p.g.m. 2 the embryo sac contained 20 to 30 antipodal cells at its chalazal end. The antipodal cells subsequently became highly polyploid and some eventually contained up to 200 times as much DNA as haploid egg nuclei. At 20 °C the sperm nuclei reached the egg and polar nuclei about 40 min after pollination. The primary endosperm nucleus divided about 6 h after pollination while the zygote did not divide until about 22 h after pollination. The endosperm often contained 16 mitotic nuclei 24 h after pollination. The nuclear division cycle during the first five division cycles was about 4.5 h. Until the tenth division cycle when the endosperm became a cellular tissue, development of endosperm nuclei was synchronous, but thereafter synchrony was progressively lost. Early embryo development was marked by a gradual decrease in the durations of successive cell cycles. This decrease was apparently correlated with a decrease in the size of embryo cells and their nuclei. Nuclear and cellular developmental rates at 20 °C were very variable. Estimates of nuclear cycle times ranged from about 60 h in the microspore to about 4.5 h in some endosperm nuclei. Nuclear volume was also very variable and ranged from about 240 μm3 for sperm nuclei in mature microspores to about 160000 μm3 in some polyploid antipodal cells. Both the wide range of nuclear types described, and the speed with which nuclear characters changed, illustrates the remarkable plasticity of the wheat nucleus which may occur in several very different forms. A comprehensive and integrated study of development at the cellular level in reproductive tissues of a higher plant species is presented. The importance of this study is twofold. First, it allows the comparison of reproductive cell behaviour in a higher plant species and in those animal species which have been intensively examined. Secondly the availability of a description of ‘normal’ development under controlled conditions in euploid plants of Chinese Spring opens the way for comparative studies using the wide range of available mutant or chromosomally different genotypes of Chinese Spring which are known to vary in their reproductive development.