Origin And Evolution Of Weedy Rice Revealed By Inter-Specific And Intra-Specific Hybridizations In Rice

Weedy rice refers to all the unwanted plants or populations of Oryza species growing inpaddy fields and/or around, which shares characteristics of both wild and cultivated rice.Weedy rice naturally grows and breeds, with various agronomic and biologicalcharacteristics, generally has taller plant height, more tillers, and shows wild traits such asearliness, easy shattering, longer seed dormancy period etc.. Its pericarp color is mostly red,so weedy rice is also called red rice. Weedy rice usually has strong stress tolerance, such ascold tolerance, waterlogging tolerance, drought tolerance, salinity tolerance, diseaseresistance and so on. The excellent feature makes weedy rice more suitable to grow andreproduce in wild environment and put it at an advantage in competition with cultivatedrice. Currently, weeds rice has become the most serious weed which brings harm to therice-growing countries in temperate and tropical regions all over the word. Yet there arelots of debate and hypothesis about the origins and evolution of the weedy rice so far. Thisstudy aims to directly validate and demonstrate the main path of the weedy rice origin,through hybridizations of inter-specific, and inter-subspecies and inter-varieties in rice,investigating and analyzing the main agronomic characteristics of cross-parents as well asthe frequency of occurrence, types, main characteristics and the trends of weedy rice-likeplants in different generations of hybrid offsprings, under two conditions of the baggedisolation and natural pollination. Meanwhile, this study attempt to reveal the modes andreasons of the weedy rice origin, through genetic component analysis of cultivated riceparents and hybrid offspring groups, using SSR, SSILP, InDel and Rc gene markers of redpericarp. The main findings were summaried as follows:1Testing of main agronomic traits of the cross-parents indicated that cultivated ricevarieties mostly carried features with yellow glume, white pericarp，awnless, compactpanicle and difficult shattering. Whereas wild and weedy rice lines showed brown orgolden color glum with or without awns, red pericarp, spreading panicles and easyshattering.2Indica-japonica differentiations of the cross-parents in cultivated rice based onmorphological classification and molecular markers were consistent. Indica-japonica constitutions of cultivated rice in9311and Nipponbare were almost geneticallyhomogeneity as revealed by SSR, SSILP, and InDel molecular markers. However, genomicconstitutions of some indica and japonica varities were genetically heterogeneity,containing various degrees of japonica components in indica varieties Diantun502,Yunhui290, Minghui63, Guichao2and Haomuxi, while consisted of certain indicacomponents in japonica variety Ansanbyeo.3The F₂progenies derived from inter-specific hybridizations among cultivated riceand its wild relatives, would be prone to produce more weedy rice-like plants. Thefrequencies of occurrence of weedy rice-like plants in F₂population varied depending onthe genetic similarities between the two parents. The more different between two parentallines are, the more frequent weedy rice like plants produced in their progenies. Thesequence is as: weedy rice or wild rice/Japonica （44.16%）＞weedy rice or wild rice/Indica（27.84%）＞Indica/Japonica （4.17%）>Indica/Indica （1.26%）＞Japonica/Japonica （0%）.4Weedy rice-like plants could be segregated from the progenies of cultivated riceinter-subspecific and inter-varietal hybridizations （seven indica/japonica and fourindica/indica combinations）. Moreover, the study showed that the frequency of theoccurrence of weedy rice-like derived from indica/japonica hybridizations was greaterthan that of indica/indica hybridizations. Meanwhile, the occurring frequency of theplants, with weedy rice-like characteristics, among F₂-F₄populations tended to increasewith the aggrandizement of generation in indica-japonica inter-subspecific hybridizationsor in indica inter-varietal hybridizations, the lowest was1.26%in F₂, and the highest wasup to15.35%in F₄. Both inter-subspecific and inter-varietal hybridization leaded to theoccurrence of weedy rice, which might be very closely related to the genetic background ofthe parents. The bigger the genetic distance between indica and japonica subspecies was,the higher the occurrence frequency of the weedy rice among the hybridization progenieswas.5Additionally, the weedy rice-like plants, segregated from the progeny （F3） of indica/japonica combination （such as Ansanbyeo/9311） were mainly indica plants （52.8%）.Whenever indica rice crossed Ansanbyeo, a japonica variety from South Korea, as paternalor maternal parents, the weedy rice like-plants was easy to be emerged among the hybridprogenies, which might be related to higher genetic heterogeneity on its indica andjaponica components. In this study, the weedy rice-like plant was not found amongjaponica/japonica derived progenies, implying that the weedy rice might not be easy tooccur from japonica hybridizations among japonica varieties. 6All cross-parents of cultivated rice varieties with white pericarp did not carry Rcgene which showed red pericarp, they were found to be rc homozygote （with14bp deletionat Rc）. Red pericarp of the weedy rice-like plants in F3progenies derived fromindica-japonica cross （Ansanbyeo/9311） was emerged through recombinant of Rc and Rdgenes, but segregated to be rc/rc or Rc/rc genotypes in its next generation.Over all results of this study would help to clearly clarify the phylogeny relationshipsof wild rice, weedy rice and cultivated rice, reveal the molecular mechanism of the maintraits in weedy rice, and put a new insight into the control and utilization of weedy rice.