37 Cactus Cytology, Genetics and Molecular Biology Anath Bandhu Das Cytogenetics Laboratory, Regional Plant Resource Centre, Nayapalli, Bhubaneswar 751015, Orissa, India Correspondence: a_b_das@hotmail.com, drabdas@yahoo.com Keywords: biotechnology, chromosome, cp DNA marker, DNA markers, genome size, GISH, isozyme markers, karyotype, RAPD ABSTRACT Recent advances in molecular biology have opened up unlimited possibilities for the construction of molecular linkage maps and for gene transfer across sexual barriers. A basic requisite in such work is a precise understanding of chromosome numbers and morphology. Cacti are a very important group of ornamental plants that require little attention to grow and have ample scope in genetic improvement through crossing for development of new cultivars or varieties with a lucrative market value. As a prerequisite for any breeding work, I provide a current update of meiotic and mitotic chromosome numbers in ~385 species belonging to 57 genera of cacti of the family Cactaceae. Chromosome numbers of n = 11 with 2x, 3x, 4x, 5x, 6x, 8x and greater polyploidy Opunctia showed a high level of ploidy despite the small chromosomes. Genome size estimates of 145 species of cacti through cytophotometry and flow cytometry (FC) are reported with a calculated value of nucleotide base pairs. 4C DNA content of Astrophytum, Ferocactus, Echinopsis, Gymnocalycium, Mammillaria, Melocactus, Parodia and Rebutia were determined with fulgen cytophotometry while Opunctia 2C DNA estimates were determined by FC using fluorescent dyes. To establish molecular phylogenetic relationships among different tribes and subtribes of cacti genetic markers like, isozymes, proteins, RAPD, cpDNA markers, rbcL gene were used to suplement the taxonomic classifications in large groups (Mammillaria, Astrophytum, Echinopsis, Ferocactus, Molocactus, Parodia and Rebutia). To established introgession of genetic materials of colour inheritance genomic in situ hybridization (GISH) was used to interpret intergeneric hybrids of cacti. This chapter provides vital information on cactus cytology, isozyme markers, DNA markers and molecular cytogenetic markers which can be used for a cactus breeding programme for intergeneric and interspecific hybrid development, particularly for the introduction of novel colours and other traits that would enrich its horticultural value. 1. INTRODUCTION Cacti, a typical group of xerophytic plants, belong to the family Cactaceae evidenced by a thick fleshy stem, spines in place of leaves, a thick epidermis with a heavy cuticle, and sunken stomata as a result of a very lengthy process of adaptation during evolution (Slaba 1992). The Cactaeae is a large family comprising of 87 genera with 2000 species; the plants are distributed mainly in the warmer parts of North, Central and South America, naturalised in Africa, Madagascar and Sri Lanka (Heywood 1985). The family is represented by perennial xeromorphic trees, shrubs or shrublets of distinctive appearance with determining character of identification for the whole group is the presence of areoles, a feature possessed only by cacti (Slaba 1992). The infinite variations in shape, size and colour of areoles and spines (Fig. 1) make the plants look curious and interesting (Das and Panda 1995). To taxonomists, it represents a great problem, being apparently still in a state of active evolution and resisting the standard herbarium procedures based on dried specimens (Heywood 1985). The Cacta- ceae have no close relatives and for a long time efforts have been made to fit them neatly into systems of classification. Now they are well sited in Caryophyllales alongside Phytolaccaceae, Portulacaceae and Mesembryanthemaceae with which they share many less obvious features including the pigment betacyanin. ® Fig. 1 A group of hybrid cacti grown in the cacti garden at Regional Plant Resource Centre (RPRC), Bhubaneswar, Orissa, India.