Eight maize lines and hybrids were used as female and crossed by tetraploid Tripsacum dactyloides L. "Huey". From these crosses, five maize stocks [Strawberry Pop, Ladyfinger Pop, 311255, Aho (Alexander High Oil), and Mo20W/Aho] showed good seed set (over 50 viable kernels per ear). The other three stocks, Tom Thumb, Mo20W and a synthetic tetraploid, showed normal early kernel development but gave nonviable, collapsed mature kernels. Viable kernels from the above crosses were germinated, root tips were fixed for chromosome counting, and seedlings were transplanted into 10 inch pots. Fresh young and mature anthers from these plants were examined by the acetocarmine-haematoxylin staining technique. The results are summarized as follows:

1. Exposed mature anthers failed to dehisce; no pollen shedding was observed.

2. Microspores from single, fresh young and mature anthers showed clear size variation.

3. In most cases, microspores from single mature anthers were aborted or degenerated, as determined by their non-staining cell contents and transparent appearance.

4. Development after meiosis was delayed and developmental differences among the microspores were observed. These variations occurred at various developmental stages during pollen development.

5. After meiosis, the genomes of maize and Tripsacum may not fuse together to form a single nucleus. Microspores that carry one large and one small nucleus are frequently seen. This phenomenon may represent a tendency of maize and Tripsacum chromosomes to fail to aggregate after division.

6. After meiosis, microspore development can be classified as (a) uninucleate, (b) first mitotic stage, (c) binucleate, (d) second mitotic stage, (e) trinucleate, or (f) mature pollen. From one young anther, the following counts were obtained:
1000 aborted or degenerated
11 tiny aborted or degenerated
40 uninucleate (a)
7 binucleate (c)

In addition to aborted or degenerated spores, the following counts were obtained from a mature anther:
62 uninucleate (a)
26 first prophase (b)
14 first metaphase (b)
16 first anaphase (b)
5 first telophase (b)
5 binucleate (c)

No microspores reached the second mitotic stage in either anther. Therefore, no mature pollen is present when the anthers are mature.

7. Microspore degeneration and delayed microspore development cause complete male sterility in maize x Tripsacum hybrids.

8. No clear starch accumulation was observed in the microspores, but many oil-like droplets of various sizes accumulated in the cytoplasm.

Degeneration and delayed development of the microspores can be attributed in part to unequal distribution of genetic material after meiosis. The hybrids carry 46 chromosomes (10 from maize and 36 from Tripsacum), which represent one complete set of maize chromosomes and two complete sets of Tripsacum chromosomes. Although they carry two different sets of metabolic assembly lines, F1 plants can survive and grow even though these assembly lines may be working independently. During meiosis, the maize chromosomes are divided randomly between two daughter cells (or lost). The frequency at which a single cell receives a complete set of maize chromosomes should be (1/2)¹⁰ = 1/1024 (or about 0. 1%). However, all the cells will receive one complete set of Tripsacum chromosomes. Therefore, all the meiotic products should function properly. In our experience, no single microspore reaches maturity, which may imply that unequal distribution of maize genetic material may cause high interference with normal metabolic processes during microspore development.

A significant goal in the study of pollen development is finding genes that control microspore degeneration (West and Albertsen, MNL 59:87) or cause delay in development. Such results can help us understand the genetic control of male sterility.

Ming-Tang Chang and J.B. Beckett
 
 

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