High Transmission Frequency of a Tripsacum Chromosome in Corn.
نویسنده
چکیده
A M O N G the progenies of advanced corn-backcross generations of hybrids of 2n Zea mays L. X 2n Tripsacum dactyloides L., several 21-chromosome stocks have been isolated which contain the 20 normal chromosomes of corn plus an extra chromosome from Tripsacum. In one of these, the extra chromosome has been transmitted through the egg in 91 percent of the 80 offspring which have been grown and checked cytologically for chromosome constitution, This is a surprisingly high frequency. There are a number of conceivable explanations with related precedents for high transmission of this sort, in addition to the possibility of parthenogenetic development of unreduced eggs. The manner of development of the female gametophyte in corn from a single megaspore, normally the basal cell of a linear quartet, provides an obvious means if the extra chromosome can somehow be included in the functional megaspore at disproportionately high frequency. SINGLETON and MANGELSWRF ( 1940) and RHOADES ( 1942) found no deviation from basal spore functioning in studies of gametic lethals and nonrandom segregation. RHOADES (1952), however, showed that abnormal chromosome 10 may be included in the basal megaspore 70 percent of the time because of neocentric activity of the terminal knob. KAYANO (1 957) working with Lilium and RUTISHAUSER (1960) working with Trillium have found that lagging supernumerary or fragment chromosomes are found most often near that end of the anaphase I spindle where the cell destined to produce the egg will be located. This behavior is apparently not typical of corn trisornes for if it were, EINSET (1943) should have found more frequent transmission of shorter chromosomes instead of the reverse, since frequency of lagging at anaphase I was thought to be inversely related to relative chromosome length. AVERS (1954) reported a tendency for precocious division of univalents in aster microsporocytes, with evidence that these univalents regularly separate equationally twice during meiosis so that high transmission frequency results. In contrast, corn univalents tend to be retarded in division, and there has been no evidence that equational separation ever occurs more than once during meiosis. Differential viability of spores, gametes or zygotes provides another basis for distorted transmission frequencies. Although ordinarily selection acts against the
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ورودعنوان ژورنال:
- Genetics
دوره 48 9 شماره
صفحات -
تاریخ انتشار 1963