Haploids and monosomics are produced by maize plants containing telocentrics for both arms of chromosome 3 and B chromosomes
--L'Heureux, TA; Muzumdar, DA; Schneerman, MC; Weber, DF

Rose and Staub (MNL 64:94-95, 1990) discovered a centromere breakage event in maize that produced stable telocentric chromosomes for both arms of chromosome 3. They also indicated (K. L. Rose and R. W. Staub, Carleton College, personal communication) that in the presence of B chromosomes, one or the other of the telocentric chromosomes was sometimes not transmitted to the zygote through the pollen due to a non-disjunctional event at the second microspore division. When such gametes (that possess only one of the two telocentric chromosomes) are fertilized by normal haploid gametes, plants containing one normal homolog plus a telocentric for one of the two chromosome arms are formed. Such an individual is a monotelodisomic (p. 4, G. Khush, Cytogenetics of Aneuploids, Academic Press, NY, 1973).

A chromosome 3 double-ditelocentric stock (DDT-3, having two copies of the 3S telocentric, two copies of the 3L telocentric, and no normal chromosome 3) was generously provided to us by K. L. Rose and R. W. Staub. We examined several plants of this stock cytologically and confirmed them to be double-ditelocentrics for chromosome 3. These were crossed as male parents by Black Mexican sweet corn that contained about 10 B chromosomes. The F1 progeny (that contained a normal chromosome 3, a telocentric for 3S, a telocentric for 3L, and about 5 B chromosomes) were crossed as male parents by female tester parents with recessive seedling-expressed mutations in either the short (g2 or d1) or the long (lg2 and et1) arm.

Numerous progeny were recovered from these crosses that expressed each of the mutant phenotypes, and we have examined meiotic cells from some of these plants. From a cross where the F1 was crossed as a male parent to a g2/g2 female parent, two different plants expressing the g2 phenotype were found to possess one normal chromosome 3 plus a telocentric for the long arm of chromosome 3. Also, from a cross where the F1 was crossed as a male parent to a lg2 et1/lg2 et1 female parent, two different plants expressing the lg2 and et1 phenotypes were found to contain one normal chromosome 3 plus a telocentric for the short arm of chromosome 3. Thus, we have recovered and cytologically confirmed monotelodisomics for both arms of chromosome 3. We have used these to map the position of the centromere of chromosome 3 on the maize RFLP map (reported elsewhere).

Previously, Chi, Fowler, and Freeling (MNL 68:16, 1994) reported that they crossed a plant carrying telocentrics for 3S and 3L as a male to female parents which carried mutants on the short arm (heterozygous for v19) or the long arm (homozygous for y3) of chromosome 3, and recovered plants that expressed these mutants. They analyzed these utilizing RFLP probes, and in all cases, they indicated that the predicted number and intensity of bands was consistent with the plants being monotelodisomics for 3S and 3L. However, they did not indicate if the plant carrying telocentrics for 3S and 3L contained B chromosomes or if it was homozygous or heterozygous for the telocentrics.

We also crossed DDT-3 plants as female parents with W22 plants that contained approximately 10 B chromosomes, and the F1s were crossed as male parents to lg2 et1 female parents. Forty-nine of 3809 (1.29%) seedlings germinated in a sandbench expressed both the lg2 and et1 phenotypes. We have cytologically analyzed root-tips of seven of these to date. Two were found to be diploids (which could have been produced if the tester was fertilized by contaminating tester pollen), one was a haploid with no B chromosomes, three were monosomics with no B chromosomes, and one was a monotelodisomic for the short arm and contained 2 B chromosomes. The haploid was a maternal haploid because it expressed the lg2 and et1 phenotypes; however, the origin of the monosomics is less clear. Thus, this system not only produces monotelodisomics, it also produces haploids and monosomics. We are analyzing additional progeny to further characterize this interesting genetic system. 

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