Russian dandelion is a sexually reproducing diploid (2n = 2x = 16) species (Warmke, 1943) of the Asteraceae family (Whaley and Bowen, 1947), native to southeastern Kazakhstan (Hellier, 2011). Cross-pollination is enforced by a self-incompatibility (SI) system (Warmke, 1943), which is likely controlled sporophytically because this mechanism has been identified in other species within the family (Brennan et al., 2011; Hiscock, 2000). The dandelion produces large amounts of high-quality rubber in its roots (Whaley and Bowen, 1947) and is adapted to temperate climates, making it a promising candidate rubber crop for southern Canada and the northern United States. Limited breeding has been conducted with this species, so development of an economically viable crop requires improvement of many traits, such as rubber yield and plant vigor.
Male sterility is characterized by a lack of functional pollen and has been identified in over 600 species including many agriculturally important plants such as maize (Zea mays), rice (Oryza sativa), canola (Brassica napus), and sorghum (Sorghum bicolor) (Kaul, 1988). It can be inherited nuclearly, in genic male sterility (GMS), or maternally, through the mitochondrial genome, in CMS. Mutations in genes involved in anther and microspore development are the primary cause of GMS (Wilson and Zhang, 2009; Zhang and Wilson, 2009; Zhang et al., 2011). CMS genes are often created by nonhomologous recombination events in the mitochondrial genome and can contain fragments of several mitochondrial open reading frames (Horn et al., 2014). Their functions are not well understood, but they are generally thought to cause gamete abortion through disruption of mitochondrial activity or aberrant programmed cell death (Chen and Liu, 2014). However, some genes produced homeotic phenotypes, where stamens are replaced by other reproductive organs, suggesting the cytoplasm may affect expression of nuclear genes (Linke et al., 2003).
Interactions between nuclear and mitochondrial genomes typically determine whether a plant is male sterile or male fertile. Nuclear Rf loci have coevolved with CMS genes and often encode RNA-binding proteins that target CMS transcripts for degradation, therefore masking the effects of the sterility-inducing cytoplasm (Chen and Liu, 2014).
Male sterility is a valuable tool for studying the mechanisms involved in anther development and gametogenesis. However, CMS most importantly can be used to facilitate hybrid seed production, which gives rise to plants with increased vigor and up to 50% greater yield compared with open-pollinated cultivars (Tester and Langridge, 2010). Both sterility (S)- and fertility (F)-inducing cytoplasms are required. Generally, a three-line system is used where a male sterile A line (S)rfrf is inbred with an isonuclear, fertile, B line (F)rfrf. After sufficient inbreeding, the A line is crossed to a genetically distinct, fertile inbred line, R. In species, such as canola, where the seed is the valuable product, the R line must be homozygous dominant at the Rf locus, (F/S)RfRf, to restore fertility to the hybrid cultivar. In russian dandelion, however, cultivar fertility is not important because the root is the harvested portion. Therefore, a commercial hybrid can be produced by crossing an (F)rfrf genotype with the A line.
GMS, in contrast, is not practical for hybridization because sterility is not maintained during inbred development (Chen and Liu, 2014). GMS that is affected by climatic conditions, termed environment-sensitive GMS (EGMS), however, can be used for hybrid production because the male sterile line can be inbred under conditions inducing fertility and then crossed under conditions inducing sterility. Photoperiod and temperature-sensitive GMS lines account for ≈20% of seed production in China (Huang et al., 2014).
Male sterility was discovered previously, in France, in diploid Taraxacum species belonging to the Erythrosperma and Ruderalia sections of the Asteraceae (van der Hulst et al., 2004). Genetic analyses with male fertile diploid and triploid plants indicated that the sterility was inherited cytoplasmically and dominant alleles at two Rf loci could restore fertility. Russian dandelion is classified in the Macrocornuta section of the Asteraceae (Richards, 1968); therefore, newly discovered male sterility in this species could have evolved independently from the germplasm discovered in France. The trait has also been identified in Taraxacum gilliesii, native to South America; however, inheritance is undetermined (Uhlemann et al., 2004).
The objective of this experiment was to determine the inheritance of the male sterile phenotype identified in two full-sib russian dandelion families. Crosses were performed to create F2, F3, and backcross (BC1) generations. Analysis of segregation ratios indicated that the sterility-inducing factor was cytoplasmically inherited and presence of a dominant Rf allele could restore fertility.
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