Study On The Genetic Variation In Rice F₂ Populations Generated From Reciprocal Hybrids At Different Altitudes

The indica-japonica differentiation is the main trend in Asian cultivated rice. Considerable differences between the two subspecies in morphology, physiology and biochemistry, and at the molecular level on nuclear as well as cytoplasm, are associated with their adaptability to different ecological environments. Genetic segregation distortion are commonly seen in progenies of crosses between indica and japonica varieties in which have obviously reproductive isolation. It is because that gametic selection or gametic abortion was carried out by gametic gene (ga) and hybrid sterile gene (S) located in the nuclear, which is controlling some physiological process. In addition, abnormal physiological metabolism also seen in some inter-subspecific crosses due to distorted expression resulted from incoordination of interaction between nuclear and cytoplasm, such as cytoplasmic male sterility (CMS). However, it was restorted by nuclear restorer of fertility (Rf) gene. Therefore, study on the genetic segregation of Rf gene under cytoplasmic backgrounds donated by indica and japonica rice, could be reference for studies of indica-japonica differentiation because that it is a typical locus on indica-japonica differentiation.The genetic parameters of morphological characters, cold tolerance at booting stage, marker genetic segregation and genetic differentiation were analyzed in reciprocal F2 populations generated at three different altitudes of 400m, 1 860m and 2 200m. The material parents are Xiaomagu, japonica rice landrace grown at Yunnan alpine region at 2 650m, and Nan34, an improved line that cytoplasm donated by indica rice. The effects of the environmental temperature induced by altitude on the F1 gametic genotype selection and on the genetic variation in their progenies under indica-japonica cytoplasmic backgrounds was studied. The results could be reference for discuss the effects of global temperature change on genetic variation and genetic diversity in rice as well as other gramineous crops. The main results were as follows:(1) The F1s under japonica cytoplasmic backgrounds had the fertility significantly higher than the F1s under indica cytoplasmic backgrounds at all altitudes. Especially, pollen fertility was much different between the reciprocal hybrids. It indicates that cytoplasm has effects on fertility, especially on pollen fertility. Beside cytoplasm, altitude had significant effects on pollen fertility of the F1 hybrids, but the effect was not obvious on spikelet fertility. The cytoplasmic backgrounds and altitude variation had selecting pressure on pollen genotype of F1s, and such fertility difference could result in genotype selection on descendant progenies.(2) Three traits, effective panicles, panicle length, and spikelet fertility, were significantly distorted toward Nan34 in all the populations, while flag-leaf width and panicle node length were significantly skewed toward Xiaomagu. Beside these traits, other three ones, total grains per panicle, plant height, and heading days, showed different favoring parents in different populations. The effect of cytoplasm on the morphological characters in F2 populations is increasing with the rise of altitude, and it is the most outstanding at 2 200m altitude.(3) Spikelet fertility was the most sensitive characteristics, could be used as indicator of cold tolerance evaluation at booting stage. The average spikelet fertiltiy of F2 (Xiaomagu×Nan34) population, under low temperature, extremely significant higher than its reciprocal F2 population generated from same altitude. In the same cross, the average spikelet fertiltiy of F2 populations generated from 2 200m are higher than those generated from other altitudes, and it is significant or extremely significant higher than those generated from 400m altitude. The results demonstrated that the progenies generated from the hybrid F1s pollinated at high altitude had stronger cold tolerance than those pollinated at low altitude. The distribution of spikelet fertility under low temperature conditions mainly controlled by cold tolerance gene(s) in F2(Xiaomagu×Nan34) populations, however, it is also strongly affected by indica cytoplasm in RF2 populations.(4) The 10 markers distributed on 7 chromosomes were significantly distorted from the expected 1:2:1 or 3:1 segregation ratio (P<0.05 or P<0.01). Seven markers were distorted in the populations generated at 400m altitude, while four and five ones were distorted in the populations generated at 1860m and 2200m altitudes, respectively. All the markers that showed distortion in the populations possessing indica cytoplasm were distorted toward Nan34, while those showed distortion in the populations possessing japonica cytoplasm did not show special favoring type. The genetic segregations of M45461, which is located within restoring gene Rf-1 locus, were related to cytoplasmic backgrounds of heterozygotes that provided male gametes. M45461 was seriously distorted towards indica parent or the one possessing indica genetic components in the populations with indica cytoplasmic background, but the marker was segregated with normal ratios in populations with japonica cytoplasmic background. The heterozygotes that provided male gametes had normal pollen fertility under japonica cytoplasmic backgrounds, but they had only around 50% fertile pollens under indica cytoplasmic backgrounds, although they all had normal spikelet fertility. The results indicated that the segregation distortion of M45461 was correspond with the male gametes but not with female ones.(5) The six F2 populations were divergent into two major groups based on cytoplamic backgrounds donated by indica and japonica rice. One group was composed by the three F2 populations generated from the cross of Xiaomagu×Nan34 at different altitudes. Another group was composed by the three F2 populations derived from the reciprocal cross at different altitudes. In both groups, the genetic divergences between populations generated at high altitude (2200m) and low altitude (400m) were larger than between those generated at middle altitude (1860m) and high altitude and or between those generated at the middle altitude and low altitude. The results also indicate that cytoplasmic effects on genetic differentiation are modified by altitude variation, especially under indica cytoplasmic backgrounds.(6) The genetic variation of rice F2 populations obviously affected by altitude due to gametic selection of heterozyote, in this study, was provided for the first time at home and abroad demonstrated by genetics and molecular biological manipulations. The results also indicated that indica cytoplasm is not well compatible with japonica nuclear background, while japonica cytoplasm does not have such trouble with indica nuclei. The results could be reference for studies of indica-japonica differentiation, and for female parent selection in breeding involved indica and japonica parents.