R. A. Porter

A comparative map of wild rice (Zizania palustris L. 2n=2x=30).

Abstract We present the first genetic map of wild rice (Zizania palustris L., 2n=2x=30), a native aquatic grain of northern North America. This map is composed principally of previously mapped RFLP (restriction fragment length polymorphism) genetic markers from rice (Oryza sativa 2n=2x=24). The map is important as a foundation for genetic and crop improvement studies as well as a reference for genome organization comparisons among species of Gramineae. A comparative mapping approach with rice is especially useful because wild rice is grouped in the same subfamily, Oryzoideae, and no other mapping comparison has yet been made within the subfamily. As rice is the reference point for mapping and gene cloning in cereals, establishing a consensus map within the subfamily identifies conserved and unique regions. The genomes of wild rice and rice differ in total DNA content (wild rice has twice that of rice) and the number of chromosome pairs (wild rice=15 versus rice=12). The wild rice linkage map reported herein consists of 121 RFLP markers on 16 linkage groups spanning 1805 cM. Two linkage groups consist of only two markers. Colinear markers were found representing all rice linkage groups except #12. The majority of rice loci mapped to colinearly arranged arrays in wild rice (92 of 118). Features of the map include duplication of portions of three rice linkage groups and three possible translocations. The map gives basic information on the composition of the wild rice genome and provides tools to assist in the domestication of this important food source.

Survival of Zizania embryos in relation to water content, temperature and maturity status

The interactions between water content and temperature on freezing and desiccation damage were examined for Zizania (wild rice) embryos at several stages of maturity. The water content of excised embryos was manipulated by flash drying at 35°C or room temperature to between 2.5 g H 2 O/g dw and 0.05 g/g. Embryos were then exposed to temperatures ranging from 5 to −50°C. Viability following the drying and cooling treatments was assayed by leakage of electrolytes and germination in culture. Viability of embryos decreased when embryos were dried below a critical water content. The critical water content was greatest for the least mature embryos. Critical water contents were also temperature dependent and increased with decreasing temperature. Even though the critical water content varied with developmental status and temperature, the water activity corresponding to the critical water content appeared to be constant at 0.90. The most mature embryos survived temperatures as low as −50°C while the least mature embryos survived only to −18°C. These trends were predicted by ‘phase diagrams’ based on the physical properties of water in embryos at different stages of maturity (Vertucci et al. , 1994a). Our results confirm the earlier prediction that long term preservation of Zizania grains is possible at −20°C and the degree of success will be related to the maturity status of the embryos.

Comparative genetics at the gene and chromosome levels between rice ( Oryza sativa) and wildrice ( Zizania palustris).

Abstract Using comparative genetics, genes, repetitive DNA sequences and chromosomes were studied in the Oryzeae in order to more fully exploit the rice genome sequence data. Of particular focus was Zizania palustris L., n = 15, commonly known as American wildrice. Previous work has shown that rice chromosomes 1, 4 and 9 are duplicated in wildrice. The Adh1 and Adh2 genes were sequenced and, based on phylogenetic analyses, found to be duplicated in wildrice. The majority of the sequence diversity in the Adh sequences was in intron 3, in which were found several MITE insertions. Cytological and molecular approaches were used to analyze the evolution of rDNA and centromeric repetitive sequences in the Oryzeae. In wildrice, copies of the 5S rDNA monomer were found at two loci on two different chromosomes near the centromeres, as in rice. One nucleolar organizer region (NOR) locus was found adjacent to the telomere, as in rice. RCS1, a middle repetitive sequence in rice, was present in all of the centromeres of wildrice. RCS2/CentO, the highly repetitive component of Oryza sativa L. centromeres, was conserved in eight of the Oryza species examined, but was not found in wildrice. Three other middle repetitive centromeric sequences (RCH1, RCH2/CentO and RCH3) were also examined and found to have variable evolutionary patterns between species of Oryza and Zizania.

Genetic dissection of seed shattering, agronomic, and color traits in American wildrice ( Zizania palustris var. interior L.) with a comparative map.

Abstract.A comparative map of American wildrice (Zizania palustris var. interior L.) was used to identify loci controlling seed shattering, plant height, maturity, tiller number, plant habit, panicle length seed length, and color traits. Two to six significant quantitative-trait-loci (QTLs, P < 0.05) were detected for each trait evaluated, representing the first trait-mapping in wildrice. The chosen population was designed to emphasize the mapping of loci controlling the shattering trait, which is the most important trait in the management of this newly domesticated species. Three loci were detected that controlled the discretely categorized variation between shattering and non-shattering plants. Seed-shattering loci were detected and validated among the F2 and F3 generations. A multiple regression model with these three loci described 49.6% of the additive genetic variation. A genetic model with the same three loci including dominance and locus interactions predicted the shattering versus non-shattering phenotype at a success rate of 87%. The comparative map was based on mapped RFLP markers used in white rice (Oryza sativa L.) and other grass species. Anchor loci provided a reference point for the identification of potential orthologous genes on the basis of white rice mutant loci and consensus grass species QTLs. Candidate orthologous loci were identified among all traits evaluated. The study underscores the benefits of extending trait analysis through comparative mapping, as well as challenges of QTL analysis in a newly domesticated species.

Physical properties of water in Zizania embryos in relation to maturity status, water content and temperature.

Changes in the properties of water in excised embryos were measured during the late stages of grain development in two cultivars of Zizania palustris and a population of the endangered species Z. texana. The relationships between water content and water activity were determined from water sorption isotherms, measured at temperatures between 35 and 5°C and then derived for lower temperatures. The freezing and melting behaviour of water in embryos at different water contents was determined using differential scanning calorimetry. The moisture content of embryos at high water activities decreased with maturation, as did the moisture content at which freezing transitions were not observed. While the temperatures of freezing and melting transitions decreased as the moisture content of embryos decreased, there were no discernible differences among embryos at different developmental stages. The properties of water measured in maturing Zizania embryos approached those for orthodox seeds as determined from the strength of water sorption, the enthalpy of the melting transition and the moisture content at which water is unfreezable. From these data we conclude that the properties of water in recalcitrant Zizania embryos change with development to resemble those of embryos of desiccation-tolerant seeds, but that the seeds never achieve the orthodox condition. The effects of interactions between moisture content and temperature on desiccation damage, freezing damage and germination in Zizania are predicted, based on the physical properties of water reported here and the correspondence of these properties with physiological function reported for other species. The resulting ‘phase diagram’ defines possible combinations of moisture content and temperature for storage under equilibrium conditions.

Formation of Panicles and Hermaphroditic Florets in Wild-Rice

Formation of panicles and flower primordia in wild-rice, Zizania palustris L., was investigated by scanning electron microscopy (SEM). Floral development involved the formation of both staminate and pistillate floral primordia in the wild-rice spikelet. The SEM study indicated that wild-rice shared a similar developmental process in panicle formation with that of cultivated white rice (Oryza sativa L.). Variations of hermaphroditic floret formation and sex expression were observed in the transition zone between the male and female portions of the panicles in several different wild-rice populations. The frequencies of plants with hermaphroditic floret formation ranged from 27% in the Pistillate population (Z. palustris L.) to 70% in the Peterson Pond (Zizania aquatica L.) population. There was a high degree of phenotypic variation in sex expression in the transition zone of panicles from different wild-rice populations.

Maintaining Food Value of Wild Rice (Zizania palustris L.) Using Comparative Genomics

Wild rice (Zizania palustris) is a naturally-occurring, aquatic plant species that is important to wildlife, aquatic biological systems and humans. Populations of Z. palustris across the geographic range continue to decline in their natural habitat. In some cases, natural populations are being lost. Sometimes referred to as American wild rice, it is genetically similar to Oryza sativa, or cultivated Asian rice. This similarity coupled with modern advances in rice genomics have allowed for comparative genetics and genomics studies between Z. palustris and O. sativa and genetic diversity studies, which have been useful for characterizing the available natural genetic resources. The ongoing wild rice breeding program has been successful in supporting and expanding the cultivated wild rice industry. The incorporation of modern molecular genetics approaches to selection have improved the ability to breed for cultivated wild rice varieties that are more resistant to seed shattering, which has contributed to increased grain production.