Central Research Institute, Cuttack, Orissa, 753006 India
Mode of reproduction and growth habit exert a great influence on chiasma
frequency and hence genetic recombination in plants. Darlington (1937) and
Mather (1943) pointed out that perennial and cross pollinated species in
general exhibit lower chiasma frequency in comparison to their related annual
self-pollinated relatives. Since the evolutionary change in many genera is
from perennial to annual habit, most of the primitive perennial species
exhibit lower chiasma frequency in comparison to the more evolved annual
forms.
The present study of chiasmata frequency at diplotene and metaphase I in eleven diploid species of Oryza is in agreement with the above generaliations and reveals that the perennial species like O. rufipogon, O. barthii, O. australiensis, O. granulata and O. collina have lower chiasma frequency than their related annual species such as O. nivara, O. sativa, O. glaberrima and O. cubensis.
In the present study (Table 1), the chiasma frequency of 0. barthii was found to be 1.31 and 1.11 at diplotene and metaphase I stages, respectively. Das (1961) while comparing the chiasma frequency of 0. rufipogon and 0. barthii also found that the latter species has exceptionally low chiasma frequency. In genus Oryza, series sativae, 0. barthii, 0. rufipogon and 0. cubensis are the perennial wild species which are mostly cross-pollinated (Sharma 1964). The evolutionary trend in this series is towards self-pollination and annual growth habit. The chiasma frequency data
Table 1. Chiasma frequency at diplotene and Metaphase I in Oryza
species
============================================================================== Chiasmata at diplotene Chiasmata at Metaphase I Species No. of Mean Mean No. of Mean Mean PMCs Xta per Xta per PMCs Xta per Xta per studied bivalent cell studied bivalent cell ============================================================================== O. australiensis (SC 452) 13 2.33 27.96 16 1.18 14.21 O. meyeriana (SC 306) 4 1.62 19.34 20 1.51 18.21 O. officinalis (SC 308) 21 2.63 31.56 18 1.83 22.0 (SC 279) 11 2.36 28.32 22 1.96 20.82 (SC 268) 8 2.83 33.96 12 2.32 27.84 O. collina 7 1.90 22.9 14 1.58 19.1 O. barthii 43 1.31 15.8 23 1.11 13.3 O. rufipogon (SC 145) 11 2.71 28.6 29 1.82 21.9 (SC 140) - - - 29 2.54 30.5 (SC 159) 14 2.41 24.5 14 1.96 23.5 O. nivara (SC 31) - - - 18 2.40 28.8 (SC 51) 21 2.56 30.75 - - - O. cubensis 14 2.61 24.32 10 2.43 29.16 O. breviligulata - - - 8 2.48 29.76 O. glaberrima* (EC 21932) 12 3.06 36.72 16 2.63 31.56 O. sativa* (A-18) 17 2.48 29.76 14 2.47 29.70 (Tl4l) 13 4.30 51.60 22 3.31 39.88 (Norin-20) 11 2.66 31.92 20 2.41 29.92 ============================================================================== Self-pollinated, cultivated species.
It is obvious from the foregoing observations that in the genus Oryza, the general evolutionary trend is towards development of mechanisms enhancing effective genetic recombination (higher chiasma frequency) and change from perennial to annual growth habit.
References
Darlington, C. D., 1937. Recent Advances in Cytology. 2nd Ed. J & A Churchill Ltd. London.
Das D. C., 1971. Pachytene analysis in Oryza. M.Sc. Thesis. IARI, New Delhi, India.
Mather, K., 1943. Polygenic inheritance and natural selection. Biol. Rev. 18(l): 32-64.
Sharma, S. D., 1964. Interspecific relationships in the genus Oryza. Ph.D. thesis. IARI, NewDelhi, India.