Introduction

In meiosis, synapsis of homologous chromosomes occur at the pachytene stage. Here chiasma formation is followed by the process of crossing over. This leads to genetic recombination. But sometimes due to environmental factors, induced or physical stress, apomixis or genotypic abnormality; the homologous chromosomes fail to retain chiasmata for quite sometime (Darlington, 1942) and hence separate prematurely remaining as univalents. This phenomenon is known as desynapsis. It may also be attributed to gene action. (Beadle, 1933). This phenomenon has been elaborately studied in various crop plants such as chickpea, barley, Anogeissus by kumar and Sharma, 2001; Kumar and Singh, 2002 and Rao and Kumar respectively. Desynapsis increases the possibility of aneuploid formation in subsequent generation resulting in reduced seed set. Mutagens are considered to be a source of variation inducing high meiotic instability and aberrations.

The present study is aimed at the evaluation of EMS induced desynaptic meiotic behaviour in Catharanthus roseus.

Material and Method

Dry seeds of Catharanthus roseus were soaked in distilled water for 6 hours & then treated with 0.5% aqueous solution of ethyl-methane sulphonate (EMS) for 3 hours, 5 hours and 7 hours respectively. Then seeds were thoroughly washed and sown in pots to raise experimental plants. At maturity, the young flowering buds were fixed in Farmer’s fixative (1:3 acetic alcohol) for 24 hours and then preserved in 70% alcohol. Anther squashes were made in 2% acetocarmine. Meiotic chromosomes from pachytene phase onwards were observed and cytologically analysed in comparison to controls maintained separately.

Pollen sterility was determined with the help of acetocarmine in desynaptic mutants. Stained & round pollen grains were considered fertile while unstained irregular outline pollen grains were sterile.

Results and Discussion

Among the experimentation plants raised, the one developed from 5 hour 0.5% aqueous EMS treated was morphologically unhealthy and abnormal. This plant slowed reduced height, smaller leaves and late flowering.

Cytological analysis estabilished it as a strong desynaptic plant with high degree of pollen sterility.

In controls, the meiosis was normal with regular occurrence of 8 bivalents at diakinesis and metaphase-I and normal 8:8 segregation at anaphase-I.

In the desynaptic variants, a number of univalents were encountered at diakinesis and metaphase-I stage  At the metaphase-I plate the univalents were distributed more towards the pole or periphery of the spindle. Large count of bivalents leads to the equatorial position while a few number of bivalents leads to the polar distribution of univalents. (Ostergren and Vigfusson, 1953) Clumping of the chromosomes oriented at the equatorial plate predominated.

There was delay and failure in the chromosomal arrangement (univalents) at equatorial plate. Therefore, there was no clear demarcation between metaphase-I and anaphase-I. Person, 1955 coined the term meta-anaphase stage for this stage.

Univalent chromosomes at diakinesis or metaphase-I may result from low chiasma frequency, precocious chiasma terminalization or by the present of asynaptic or desynaptic genes in prophase-I (Gottschalk and Kaul, 1980).

The abnormalities encountered in the species at anaphase-I were irregular chromosome segregation, laggards and bridges Here, chromosomes showed differential migration towards the poles which leads to the occurrence of laggards. The second meiotic division also showed severe irregularities.

The desynaptic plants exhibited high sterility when compared to controls which were locally stabilized and highly fertile.

Occurrence of desynapsis might have taken place due to mutation in the gene responsible for formation of synaptonemal complex (SC). It is probable that the defective SC protein would not hold homologues together for long (Armstrong et al., 2002). It was suggested that the recombination modifier mutation in rec gene restricts recombination which leads to desynapsis.

Desynapsis causes varying degree of chromosomal aberrations and imbalance in the gametes. This raises the possibility of altered chromosome progeny. Aneuploids may arise in the subsequent generations. Desynapsis may be helpful in the genetic analysis of plants.

Acknowledgements

The Second author (Rachna Prasad) greatfully acknowledge Dr. Namita Kumari for her able guidance during the preparation of the manuscript.

References

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