Chromosome Number: The Implications of Cytogenetics

Important findings arising from cellular analyses in the early 20th century have dramatically enhanced understanding of grass taxonomy and provided a suite of characters derived from the structure and behavior of complex biomolecules. Analysis of grass chromosomes is an excellent additional taxonomic tool for grass classification. As early as the 1920s, it was recognized that chromosome number helps group grasses into taxonomically informative groups (Evans, 1926). Avdulov (1931, cited in Jauhar, 1993) examined 232 grass species and found that three basic chromosome numbers (n) existed in these grasses, although in most cases the numbers were multiples of a "basic chromosome number", as discussed later in this section. Subsequently, chromosome numbers have been documented for thousands of grasses, and other basic numbers have been identified. The typical basic chromosome numbers of the six subfamilies of grasses are: Pooideae (x = 7); Panicoideae (x = 5, 9, 10); Chloridoideae (x = 9, 10); Bambusoideae (x = 12); Oryzoideae (x = 12); and Arundinoideae (x = 6, 12). Chromosome number together with chromosome morphology constitutes the karyotype of the organism. Aspects of chromosome morphology commonly used in grass taxonomy include relative length of chromosomes, position of the centromere, presence and number of satellite DNAs, and secondary constrictions (Malik and Thomas, 1966). Although it should be noted that many exceptions to the general rule exist (Gould and Shaw, 1983), chromosome karyotyping has become invaluable in grass taxonomy.

Cytological analyses also have provided a genetic explanation underlying much of the success or failure in breeding artificial hybrids (see Chapter 19). Successful fertilization within a species results in efficient pairing of "parental" chromosomes during prophase I of meiosis. Such homologous pairing allows efficient segregation of chromosomes into daughter cells during anaphase. Theoretically, as grass populations diverge due to evolution over time they accumulate mutational differences in their chromosomal DNA sequences. When such differences are present in sufficient numbers, they prevent the efficient chromosome pairing in meiosis that is needed to produce viable gametes (sex cells). Instead, dysfunctional gametes are produced with irregular chromosome sets. Unsuccessful attempts in crossbreeding grasses to obtain fertile hybrid plants usually are characterized by such defects in meiosis.

If one attempts to crossbreed different, but closely related grass species, there often will be varying levels of pairing among related chromosomes. This phenomenon is known as homeologous or imperfect pairing, and produces partial fertility in hybrids of closely related but diverging grass species (Jauhar, 1993). Hybrid plants can persist in a vegetative state if they are perennial, but most often they are partially or completely sterile. A technique commonly used by grass breeders to restore proper chromosome pairing and fertility is to treat fertilized embryos with the chemical colchicine, which promotes a doubling of the chromosome complement. In effect, the result is a perfectly pairing homologous chromosome set, allowing the hybrid to carry out normal meiosis.

Both conventional crossbreeding and cytological analyses provide estimates of relatedness between grasses. A breeder can generate hybrid plants, examine their fertility, and correlate hybrid fertility with karyotype and chromosomal segregation during meiosis. The synthesis of these two approaches (crossbreeding and cytological evaluation) is a very powerful method for determining evolutionary relationships among grasses. This concept was described by Malik and Thomas (1967) as the "conventional genome concept," which is based on chromosome pairing in F1 hybrids created between different species. The formation of bivalents (chromosome pairings in meiosis) indicates homeology of chromosomes or chromosome segments, thus reflecting the relatedness of the grasses used.

Natural polyploids often are hypothesized to have arisen as interspecific hybrids between parental species that remain extant in nature. In such cases, the postulated parent species can be hybridized to test the origin of the natural hybrid. If such a hybrid can be produced, and if morphological features and chromosomal behavior of the experimental and natural hybrids appear similar, then this is strong evidence that the proposed parent species are indeed the ancestors of the natural hybrid.

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