Cell fusion and chromatin degeneration in an inbred line of maize

Short Communication

Cell fusion and chromatin degeneration in an inbred line of maize

Creuci Maria Caetano-Pereira¹, Maria Suely Pagliarini² and Edward Madureira Brasil³

¹CEFET-PR, Campo Mourão, PR, Brasil.

²Departamento de Biologia Celular e Genética, Universidade Estadual de Maringá, 87020-900, Maringá, PR, Brasil. Send correspondence to M.S.P.

³Escola de Agronomia, Universidade Federal de Goiás, Goiânia, GO, Brasil.

ABSTRACT

A large number of maize genotypes are currently being evaluated to determine the effect of environmental stress caused by the aluminum found in cerrado soils on meiotic behavior. In the present study we report the occurrence of cell fusion followed or not by chromatin degeneration in meiocytes of some plants of an inbred line of maize grown on acid and corrected soil in the Brazilian central plateau. The results suggest the absence of any soil effect on the irregularities observed since the number of affected plants was higher among those grown on corrected soil. The possible causes of these irregularities and their influence on male sterility are discussed.

INTRODUCTION

The meiocyte is a highly specialized cell able to produce four haploid cells after a series of genetically controlled steps. In plants, however, mutations, hybridization, and environmental stress, among other factors, may alter the constitution or the expression of genes that act during meiosis, resulting in abnormal microspores and in atypical pollen grains.

A large number of genotypes with different levels of heterozygosis grown in the Brazilian central plateau are currently being evaluated in terms of meiotic behavior to determine the effect of environmental stress caused by the high aluminum levels found in cerrado soils. In the present study we report the occurrence of cell fusion followed or not by progressive nuclear degeneration in the meiocytes of some plants of an inbred line of maize cultivated under these conditions.

MATERIAL AND METHODS

The line to be described (GO-1) is part of a lot of 40 inbred maize genotypes, 20 of them unselected and 20 selected for tolerance of high soil acidity caused by aluminum excess. These genotypes were grown on acid soils (A) in Pirenópolis (State of Goiás) and on corrected soil (C) in Goianésia (State of Goiás), both located on the Brazilian central plateau. The line in question belongs to the group of genotypes not selected for acid soil tolerance.

These lines are in an advanced stage of inbreeding within a breeding program and were originated from populations of the National Center of Research on Maize and Sorghum/Embrapa. Seven plants were analyzed per type of soil.

Male inflorescences in the ideal stage for meiotic study were fixed in Carnoy (3:1 ethyl alcoho:acetic acid), transferred to 70% alcohol and stored under refrigeration. Slides were prepared by the squash technique followed by staining with 1% propionic carmine, according to Rhoades (1950). A minimum of 500 cells per plant were analyzed, and microspores and pollen grains were counted separately. Statistical analysis were made to verify the significance of the data obtained. The analysis were performed employing a chi-square test using a contingency table, as recommended by Steel and Torrie (1960).

RESULTS

Abnormalities commonly reported for inbred lines were detected among the plants of the GO-1 line grown on both soil types, i.e., univalent chromosomes, bridges and chromosome fragments, irregular spindles, cytomixis, and early and late chromosome segregation, among others. However, one plant (4A) grown on acid soil, and three plants (3C, 4C and 7C) grown on corrected soil, exhibited uncommon abnormalities.

Progressive nuclear degeneration was observed in plant 4A (Table I), reaching a high frequency of cells and meiotic products (c² = 2.23, P < 0.01). Affected cells showed normal size or appeared as syncytes in which an increased content of chromatin could be observed despite pycnosis. The syncytes must have resulted from the cell fusions observed since the early stages of meiosis. Figure 1a shows a syncyte formed by the fusion of three cells in diakinesis side by side with an enormous syncyte with countless micronuclei. In all affected cells the chromatin material in the pycnotic condition was undergoing degeneration (Figure 1b) and the cell cycle remained incomplete.

Table I
- Percentage of cells presenting nuclear degeneration in plant 4A, grown on acid soil.

Stage	No. of cells analyzed	Cells with nuclear degeneration (%)
Meiosis I	839	64.95
Meiosis II	506	59.23
Sporads	467	56.95

Figure 1
- Some aspects of chromatin degeneration in plant 4A. a) The arrow indicates a syncyte formed by three meiocytes in pachytene, while the arrowhead indicates a larger syncyte in an advanced meiotic stage presenting countless micronuclei of different sizes. b) Chromatin degeneration in a syncyte.

Plants 3C, 4C and 7C had a different behavior. Despite of syncyte formation, the meiosis was complete. Chromatin degeneration did not occur. Microspores and anomalous pollen grains in terms of shape, size and number of nuclei were present. Statistical analysis showed differences among plants in relation to this characteristic (c² = 483.44, P < 0.01). Table II shows the frequency of anomalous meiotic products in these plants, which reached 65.79% in plant 7C. Figure 2 shows the evolution of the syncyte from prophase I to the formation of pollen grains of irregular shape. Despite the syncyte shape, in these plants the nuclei always remained separate (Figure 2a). Few normal tetrads were observed. There were dyads, triads, polyads and tetrads with microspores of irregular size or presenting microcytes. The abnormal microspores (Figure 2b) and the microcytes gave origin to pollen grains differing in size and in chromosome content (Figure 2c).

Table II
- Percentage of anomalous meiotic products in plants 3C, 4C and 7C, grown on corrected soil.

Plant	No. of sporads analyzed	Anomalous meiotic products (%)
3C	1657	26.67
4C	4527	17.08
7C	994	65.79

Figure 2
- Some aspects of the formation of anomalous meiotic products in plant 4C. a) Syncyte formed by meiocytes in diakinesis. b) Multinucleate microspore of irregular shape. c) Normal and anomalous pollen grains differing in size.

DISCUSSION

Cell fusion is a phenomenon described for some plant species (Price, 1956; Kamra, 1960a,b) which may give origin to syncytes. According to Nirmala and Rao (1996), a syncyte is a periplasmodial mass containing more than one nucleus. The nuclei occur individually or coalesced, forming a polyploid nucleus. In general, syncytes lead to the formation of abnormal pollen grains. Syncytes of different sizes with scattered bivalent groups originated by disorders in cytokinesis during the premeiotic mitoses were described by Smith (1942) in Hordeum vulgare as a result of the mu (multiple sporocytes) gene capable of forming pollen grains of various sizes and without chromatin, while multinucleate pollen grains with multiple chromosome complements originated from syncytes were described in Kniphofia (Moffet, 1932).

Cell fusion and chromatin degeneration may be caused both by environmental and genetic factors (see Nirmala and Rao, 1996). The present results suggest that the abnormalities observed were not caused by environmental factors since only one plant grown on acid soil (4A) presented nuclear degeneration in isolated cells or in syncytes, whereas three plants grown on corrected soil (3C, 4C and 7C) presented anomalous meiotic products in terms of shape, size and number of nuclei, also resulting from cell fusion but not accompanied by nuclear degeneration, as in plant 4A. The remaining plants of the line, grown either on acid or corrected soil, did not present these abnormalities.

Mutants that affect different steps of meiosis, causing diverse effects, have been described in maize (for a review, see Golubovskaya, 1979, 1989). Among mei mutants that cause syncyte formation and chromatin degradation are pam1, pam2 and pam-A344 genes (see Golubovskaya, 1989). The abnormal plants of this study showed some similarities with pam mutants. Syncyte formation was observed in all plants while chromatin degeneration occurred in the plant 4A. However, in the plants cultivated in corrected soil (3C, 4C and 7C) the meiosis was complete, leading to formation of abnormal pollen grains, which does not occur in pam mutants. So, we suggest that the present characteristic is genetically controlled, but the gene involved is not the pam gene. As the line in question is in an advanced inbreeding process due to successive self-fecundation, recessive genes causing anomalies may have been placed in homozygosis. In some species syncyte formation is a monogenic character (Pantulu and Manga, 1971; Rao and Koduru, 1978; Rao et al., 1991) associated with chromosome degeneration in some instance (Rao et al., 1991).

Another hypothesis to explain the occurrence of these abnormalities concerns the possible presence of the Mutator transposon described in maize by Robertson (1978). This transposon, with preferential activation at the end of the premeiotic mitoses and/or during the meiotic process, can elevate the mutation rate by as much as one hundred percent. Many lines originated from some populations of the National Center of Research on Maize and Sorghum/Embrapa have been showing a high frequency of meiotic mutants never before reported to occur spontaneously. Some of these irregularities have been reported by our group (Taschetto and Pagliarini, 1993; Caetano-Pereira et al., 1995a,b, 1997; Caetano-Pereira and Pagliarini, 1996, 1997; Defani-Scoarize et al., 1995a,b, 1996) while countless others are still under study.

The application of meiotic mutants for the production of male sterility has been described for countless plant species. Only in maize, Albertsen and Phillips (1981) reported 13 meiotic mutants able to provoke male sterility, among others described by Golubovskaya (1979, 1989). Partial male sterility caused by many distinct and simultaneous meiotic abnormalities in an inbred line originated from a population of the same origin as the present one has been reported by Defani-Scoarize et al. (1995b). Cell fusion at the levels observed here, followed or not by chromatin degeneration, forming anomalous meiotic products, may be considered a source of male sterility in affected plants.

RESUMO

Com o objetivo de avaliar o efeito do stress ambiental causado pelo alto teor de alumínio encontrado nos solos de cerrado sobre o comportamento meiótico, um elevado número de genótipos de milho está sendo avaliado. Neste estudo relata-se a ocorrência de fusão celular, seguida ou não de degeneração cromatínica, em meiócitos de algumas plantas de uma linhagem endogâmica cultivada em solo ácido e corrigido no planalto central Brasileiro. Os resultados não sugerem qualquer efeito do solo sobre as irregularidades observadas, pois o número de plantas afetadas foi maior dentre aquelas cultivadas em solo corrigido. As possíveis causas destas irregularidades e suas influências na produção de macho-esterilidade são discutidas.

(Received March 12, 1998)

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Publication Dates

• Publication in this collection
  02 June 1999
• Date of issue
  Mar 1999

History

• Received
  12 Mar 1998