A study of the meiotic cytology of thirteen nuclear male sterile genes

Many different nuclear male sterile (ms*) genes have been isolated in maize. In a plant homozygous recessive for such a gene, the pollen fails to develop to maturity. We are analyzing a number of nuclear male sterile genes in an attempt to determine the mode of action of these genes. It is possible that certain of these genetic factors control some portion of the meiotic process. If this were the case, plants homozygous recessive for one of these genes would have an abnormal meiotic sequence. Thus, this material might be a source of new meiotic mutants. For this and other reasons, we are analyzing meiosis in these nuclear male steriles. To date, we have analyzed meiosis in thirteen different nuclear male steriles.

The lines under analysis were generously provided to us by Dr. Earl Patterson of the University of Illinois. In each of the lines provided to us, a plant homozygous recessive for a male-sterile locus (ms*/ms*) was crossed as a female parent with a male heterozygous for the same locus (Ms*/ms*). Half of the progeny of such a cross are fertile (Ms*/ms*) and the other half are male-sterile (ms*/ms*). To date, we have examined lines segregating for homozygosity at the following loci: ms, ms2, ms3, ms5, ms7, ms8, ms9, ms10, ms11, ms12, ms13, ms14, and ms17.

Each of the lines was planted and microsporocyte samples were removed from several plants in each line. These samples were placed immediately in a fixative solution (ethanol:propionic acid, 3:1 v/v) and stored under refrigeration.

When the plants reached anthesis, each was examined to determine if it was ms*/ms* (male sterile) or Ms*/ms* (male fertile). Microsporocyte samples from fertile and male-sterile plants were analyzed cytologically using the standard propiono-carmine smear technique.

Five diagnostic stages in meiosis were compared to determine if the meiotic process was altered by a given male-sterile gene. In all male-sterile types, pairing between homologous chromosomes appeared normal at pachytene. At diakinesis, 10 bivalents were regularly present. At metaphase I, 10 bivalents were consistently present on the metaphase plate in each cell, and the bivalents separated in a normal manner at anaphase I.

The quartet stage is a powerful diagnostic stage because a study of it can show if a loss or nondisjunction of chromosome 6 took place during either the first or second meiotic division. If the disjunction of chromosome 6 proceeds normally, a nucleolus will be present in each of the four haploid meiotic products because chromosome 6 bears the nucleolar organizing region of maize. If nondisjunction or loss of chromosome 6 took place, one or more members of the quartet would lack a chromosome 6, and consequently it would lack a nucleolus. If nondisjunction took place during either division, two nucleoli would sometimes be found in one or two members of the quartet if nucleolar fusion had not taken place. Also, if chromosome loss took place, micronuclei would be found in quartet cells.

In the male-sterile lines examined, one nucleolus was regularly found in each of the four meiotic cells at the quartet stage. Thus, nondisjunction or loss of chromosome 6 did not occur during meiosis in any of the male-sterile lines analyzed. Since the disjunction of chromosome 6 was normal in these types, the disjunction of other chromosomes also presumably was normal. The fact that micronucleoli were not found in these cells supports this last conclusion.

Another type of male-sterility is cytoplasmic male sterility (CMS). In certain cytoplasmic male-sterile plants, breakdown of the tapetum is believed to be responsible for the male sterility. Tapetal cells at the quartet stage were also examined in each of the male-sterile types, and all tapetal cells appeared normal. It is possible, however, that the tapetal layer might break down later in development in some types and be responsible for the male sterility.

We are continuing our investigation of additional male-sterile types. We will also be examining post-meiotic stages in an attempt to determine the stage and mode of breakdown in each type.

If additional male-sterile types are available from any source, we would appreciate it if you could send them to us for analysis. Dr. Patterson has supplied us with lines segregating for the following genes: ms, ms2, ms3, ms5, ms7, ms8, ms10, ms11, ms12, ms13, ms14, ms17, as, po, ms*-Bear#1, ms*-Bear#2, ms*-Bear#3, ms*-Bear#4, ms*-Bear#5, ms*-Bear#6, ms*-Bear#7, ms*-Bear#8, ms*-Bear#10, ms*-Holden 4439, ms*-M11, ms*-M70, ms*-B76, at-si.

Tau-San Chou and D. F. Weber


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