42. Oryza perennis: A new source of cytoplasmic male sterility in rice

R. DALMACIO, D.S. BRAR, L.A. SITCH, S.S. VIRMANI and G.S. KHUSH

Internationl Rice Research Institute, Manila, Philippines

Most of the commercial hybrids of indica rice are based on wild abortive (WA) source of cytoplasmic male sterility. More than 95% hybrid rice grown in China has been derived from WA male cytosterile lines (Yuan and Virmani 1988). Such cytoplasmic uniformity could lead to genetic vulnerability of hybrid rice to diseases and insect pests. To overcome this problem, diversification of cytoplasmic male sterility source is essential. We crossed 45 accessions of O. perennis and 4 accessions of O. rufipogon as female parents with IR54 and IR64. Both IR54 and IR64 can restore fertility of cytoplasmic male sterile (CMS) lines possessing the WA cytoplasm. Data on pollen sterility were recorded in F1 and backcross populations. Sterile hybrids were backcrossed four times with the respective recurrent parents. The F1 hybrids showing more than 30% pollen fertility were not used in backcrossing. Sitch and Dalmacio (1989) reported variation in pollen fertility 

Table 1. Pollen sterility of F1 and backcross progenies derived from cross of
O.  perennis (Acc. 104823) with O. sativa cultivars
==============================================================================
                                   Male parent (IR64)
                               Pollen fertility (% of progeny)*      Plants
Female Parent  Generation    ====================================    analyzed
                              C     S    PS    PF     F                 (No.)
==============================================================================
O. perennis
(Acc 104823)     BC7       100.0    0     0     0     0                 30
                 BC6       100.0    0     0     0     0                 24
                 BC5       100.0    0     0     0     0                600
                 BC4       100.0    0     0     0     0                 29
                 BC3        98.6    1.4   0     0     0                291
                 BC2        84.0   11.0   3.0   2.0   0                237
                 BC1        46.1   11.8  21.1  10.5  10.5               76
                 F1          0     16.7  16.7   0    66.6                6
==============================================================================
Pollen fertility classes: CS=completely sterile, 0% pollen fertility; S=sterile, 1-10% pollen fertility; PS=partially sterile, 11-30% pollen fertility; PF=partially fertile, 31-60% pollen fertility; F=fertile, 61-100% pollen fertility.


Fig. 1. A new cytoplasmic male sterile line, IR66707A having cytoplasm of O. perennis (accession 104823) and nuclear background of IR64.

of F1s and backcross progenies in various cross-combinations.

Of all the backcrosse derivatives, one line having the cytoplasm of O. perennis Acc 104823 and the nuclear background of IR64 was found to be stable for complete pollen sterility (Table 1) and did not set any seed under selfed conditions.

The newly developed CMS line has been designated as IR66707A (Figure 1). Crosses of IR66707A with six other restorers of WA cytoplasm also show almost complete (93-100%) pollen sterility, indicating that the cytoplasm of IR66707A, is different from WA. IR66707A is now in BC7 and resembles in morphological characters with IR66707B (maintainer line) except that it is 4-5 days late in flowering. This line is completely sterile (0% seed set) under selfed conditions and the amount of outcrossing in seed production plots varies from 2.5 to 23.9%, average being 10.2%. IR66707A was tested in the phytotron at four temperatures (19/15 deg C, 29/21 deg C, 33/25 degC, 41/33 deg C) and was found to be stable for male sterility without any seed set upon selfing. We are in search of restorers of this male sterile line for use in hybrid rice breeding.

References

Yuan, L.P. and S.S. Virmani, 1988. Status of hybrid rice research and development, In IRRI (ed.), Hybrid Rice, pp. 7-24. IRRI, Manila.

Sitch, L.A. and R.D. Dalmacio, 1989. Identifying potential sources of cytoplasmic male sterility in rice. RGN 6: 125-127.