Sugarcane belongs to the genus Saccharum L., of the tribe Andropogoneae in the grass family (Poaceae). This tribe includes tropical and subtropical grasses including the cereal genera Sorghum and Zea (corn). The taxonomy and phylogeny of sugarcane is complicated as plants from five genera share common characteristics and form a closely related interbreeding group known as the ‘Saccharum complex’. The Saccharum complex comprises of Saccharum, Erianthus section Ripidium,Miscanthus section Diandra, Narenga and Sclerostachya (Daniels & Roach 1987). These genera are characterised by high levels of polyploidy and frequently unbalanced numbers of chromosomes (aueuploidy) making it difficult to determine taxonomy and resulting in many previous revisions of the taxonomic relationships (Daniels & Roach 1987; Sreenivasan et al. 1987).
1.3.2 Genus Saccharum
The Saccharumgenus comprises six species S. spontaneum, S. officinarum, S. robustum, S. edule, S. barberi, and S. sinense (D'Hont et al. 1998).
S. officinarum is thought to have resulted from the complex introgression between S. spontaneum, Erianthus arundinaceus and Miscanthus sinensis (Daniels & Roach 1987). S. officinarum has a chromosome number of 2n = 80 with a basic chromosome number (x) of 10 making this species a polyploid (having more than two chromosome sets eg octaploid, 2n=80 - eight complete sets of chromosomes). However, S. officinarum is not a simple polyploid. It is a complex hybrid of different species as it an autopolyploid (more than two sets of homologous chromosomes derived from a single species) and also an allopolyploid (possessing two or more unlike sets of chromosomes) (Sreenivasan et al. 1987). This is indicated by whole chromosomes in S. officinarum that are homologous with those in the genera Miscanthus and Erianthus section Ripidium (Daniels & Roach 1987; Besse et al. 1997).
S. spontaneum is a smaller, highly polymorphic, disease resistant, high fibre and highly vigorous species, having 2n = 40 to 128 chromosomes. It is also a complex polyploid with a probable basic chromosome number of eight or 10 (Sreenivasan et al. 1987; D'Hont et al. 1996). It can be distinguished from the cultivated Saccharum by thinner canes and a narrow panicle (Pursglove 1972). Characteristics of the spikelets at the end of the tertiary branches of the inflorescence are also used by taxonomists to help distinguish the species from other Saccharum spp.
S. barberi and S. sinense are thought to be wild species but have been in cultivation since prehistoric times in northern India and China respectively. This had lead to considerable interbreeding with other genera and species and consequently these species are thought to be ancient intergeneric hybrids (Daniels & Roach 1987). S. barberi is thought to be the product of S. officinarum x Erianthus (sect. Ripidium) introgression, while S. sinenseis though to be derived from S. officinarum x Miscanthus introgression, each containing chromosomes homologous to S. officinarum and S. spontaneum as well as to those from members the genera Erianthus and Miscanthus again indicating the complex origins and inter-relationships within the Saccharum genus (D'Hont et al. 1996; Daniels & Roach 1987).
S. robustum is a wild species thought to be an intermediate step in the evolutionary pathway between S. spontaneum and S. officinarum. Two major groups with the species are known, those that have 2n=60 and 2n=80 chromosomes respectively.
S. edule is similar morphologically to S. robustum except that the flower spike or inflorescence is compacted and it is cultivated as a vegetable in the islands of the Pacific and Papua New Guinea. S. edule is thought to be derived from introgression of S. officinarum or S. robustum with other genera (Daniels & Roach 1987). A summary of the genetic characteristics of the Saccharum species is shown in Table 1.
Table 2: Sugar cane members of genus Saccharum (Daniels & Roach 1987; Buzacott 1965).
Commercial hybrid cultivars of sugarcane descended from interspecific hybridisation between S. officinarum and S. spontaneum (Bull & Glasziou 1979). S. officinarum or the ‘noble canes’ accumulates very high levels of sucrose in the stem but have poor disease resistance. Conversely S. spontaneum accumulates no sucrose but is a highly polymorphic species with much higher levels of disease resistance, adaptability and stress tolerance (Sreenivasan et al. 1987). The basic breeding concept involves the combination and improvement of vigorous, disease resistance from S. spontaneum and high sucrose content from S. officinarum. Increasing sucrose content while maintaining disease resistance of commercial hybrid cultivars has been achieved through a number of back-crossing to several different cultivars of S. officinarum (Bull & Glasziou 1979). Consequently, S. spontaneum genetic component is reduced in commercial hybrid cultivars. Of the chromosomes in these commercial hybrid cultivars, approximately 80% are derived from S. officinarum L. and 10% are from S. spontaneum L., with remainder being chromosome from the two species produced by the natural process of synapsis during meiosis (D'Hont et al. 1996).
Interspecific hybridisation between S. officinarum as the female parent and S. spontaneum as the male parent produce progeny that have a triploid chromosome number (2n + n = 100 to 130) (Sreenivasan et al. 1987; D'Hont et al. 1996). This phenomenon, thought to arise either through endoduplication or fusion of two nuclei during meiosis (D'Hont et al. 1996), facilitated breeding of modern sugarcane varieties as the ‘officinarum’ qualities recovered more quickly in the hybrids, thus requiring fewer rounds of backcrossing to produce high sucrose varieties.
This culminated with the release of a variety called POJ2878 (‘Java Wondercane’) in 1921 in Java (Indonesia), which became an important variety, allowing for a 35% increase in production over previous best varieties (Cox et al. 2000; Jeswiet 1929). Most commercial cultivars used in Australia today can be traced to this variety (Cox et al. 2000). Sugarcane breeding for improved varieties is a time consuming process taking upwards of ten years from initial crosses to final agronomic assessment of elite varieties (Cox et al. 2000).