CYTOLOGY OF ROSE

Rose, the queen of flowers, the most universally recognized and beloved flower in the world representing love, affection, compassion, purity, innocence and passion, belongs to the family Rosaceae. The genus Rosa contain more then 1400 cultivars and 150 specie. The genus Rosa is further classify into 4 sub genera, Eu Rosa, Platyrhodon, Hesperhodes, and Hulthemia, and distributed widely throughout the northen hemisphere. The subgenus Eu Rosa includes 11 sections. The sections Caninae and Cinnamomeae are the largest and comprise about 50 and 80 species, respectively (Wissemann, 2003) Indo-Pak subcontinent has always been the sight of attraction for the whole world regarding its natural flora. About 25 species have been reported growing in this area and many of them have contributed to the development of modern ornamental roses.

The genus Rosa is usually subdivided into four subgenera, the largest of these is subgenus Rosa with 10 sections. Most of the genetically analysed rose species appear to be sexual and diploid (2n = 14) or tetraploid (2n = 28) although there are a few triploid (2n = 21), hexaploid (2n = 42) and octaploid species (2n = 56). The diploid species are usually self-incompatible whereas the polyploids are self-fertile. Pollen stainability is usually high in all species with even ploidy levels, i.e. 2x, 4x or 6x. Rose species are usually sexual and have a regular meiosis but there is one deviating section, Caninae, which harbours the so-called dogroses. Most of these are 5x but there are some taxa with 4x and 6x. Only seven chromosomes (derived from seven bivalents) are transmitted through the pollen grains, whereas egg cells contain 21, 28 or 35 chromosomes (derived from seven bivalents and 14, 21 or 28 univalents) depending on the ploidy level. Apomixis occurs occasionally in the dogroses and genetic selfing is probably common since these taxa are self-fertile. Interspecific hybridization takes place spontaneously among rose species at all ploidy levels and is used as a potent tool in plant breeding. Information about compatibility, breeding system, pollen viability, chromosome number and inheritance is important for optimal utilization of crosses in rose breeding.

The organisms with more than two genomes are called polyploids. Among plants and animals, the polyploidy occurs in a multiple series of 3, 4, 5, 6, 7, 8, etc., of the basic chromosome or genome number and thus is causing triploidy, tetraploidy, pentaploidy, hexaploidy, heptaploidy, octaploidy, respectively. Ploidy levels higher than tetraploid are not commonly encountered in natural populations, but our most important crops and ornamental flowers are polyploid, e.g., wheat (hexaploid, 6n), strawberries (octaploid, 8n), many commercial fruit and ornamental plants, liver cells of man, etc. Other examples of polyploidy among plants and animals are following.

A continuous polyploid series has been reported in rose plant. A euploid series of basic number of 7 (monoploid) including diploids (2n= 14), triploids (21), tetraploids (28), pentaploids (35), hexapolid (42), and octaploid (56) has been reported in different species of Rosa. Likewise, the genus Chrysanthemum has basic chromosome number 9 and has a euploidic series of diploid (2n = 18), tetraploids (4n=36), hexaploids (6n=54), octaploids (8n=72) and decaploids (10n=90) in its different species.

 GENE MAPPING IN ROSE:

Parental linkage maps of a segregating population of diploid rose hybrids (2n=2x =14), composed of 365 uni-parental AFLP and SSR markers, have been constructed using a population (n=88) derived from a cross between two half-sib parents (P119 and P117). Of the markers, 157 P119 markers (85 %) mapped on eight linkage groups and 133 P117 markers (78 %) on seven linkage groups. The resulting linkage maps of P119 and P117 spanned 463 cM and 491 cM with an average of interval between markers of 2.9 cM and 3.7 cM, respectively. The present genetic maps were used to identify quantitative trait loci (QTLs) for two growth vigour-related traits, leaf area and chlorophyll content, using the Multiple QTL Mapping approach. Three QTLs for leaf area and two QTLs for chlorophyll content were identified. The QTLs accounted, in total, for 50.8 % (range 7.0-23.1 %) and 25.8 % (range 7.6-18.2 %) of the total phenotypic variance for leaf area and chlorophyll content, respectively. The detection of highly significant major QTLs enables

Plant breeding using marker assisted selection is a second way to use a gene map. If you know where a gene is on a gene map, DNA markers that flank the gene of interest can be identified. Just like mileage markers on an interstate highway indicate where you are on the highway, the presence of the DNA markers would infer the presence of the desirable gene in the offspring. DNA markers can be detected soon after seed germination, as soon as a few leaves are available to use for DNA marker analysis. This process makes early detection of the presence of the gene possible even though the actual expression of the trait does not occur until much later in maturation. This is especially valuable, for example, when working with genes that control flowering in plants that take many years to bloom and fruit or traits such as disease resistance that take several years to evaluate. Therefore, DNA markers tightly linked to a gene can be used by breeders to select and cull breeding lines. Also if several genes exist for resistance to a disease, e.g., black spot, DNA markers can be used to "stack" these genes into one rose.

Additionally, once a gene is identified, its function and the biochemical steps leading to the trait being expressed can be studied and its genetic control understood.