Haploid production of potatoes by anther culture

Abstract

The major cultivated species of the potato, Solanum tuberosum L. ssp. tuberosum is an autotetraploid (2n=4x=48). Other cultivated species often used in genetics and breeding are: the tetraploid species andigena (S. tuberosum L. spp. andigena Hawkes), the diploid species (2n=2x=24) stenotomum (S. stemotomum Juz. Et Buk.) and phureja (S. phureja Juz. Et Buk.). Chacoense (S. chacoense Bitt.), a wild diploid species, is often used in genetic studies and anther culture. There are a large number of wild species with ploidy levels ranging from diploid to hexaploid (2n=6x=72). Of the 176 species surveyed by chromosome counts, 73% are diploid, 15% are tetraploid, and 6% are hexaploid. The rest are hybrid species that are triploid or pentaploid. Several major gene banks maintain the world collections of potato species and varieteis. The German-Netherlands Potato Genebank in Braunschweig, Germany, and the USA Inter-regional Potato Introduction Project have published up-to-date inventories. Systematic evaluation of germplasm collections are carried out by the International Potato Center, the German-Netherlands Potato Genebank, and the USA Inter-regional Potato Introduction Project. The tetraploid varieties can be used to produce dihaploid progenies (2n=2x=24). Dihaploids provide many advantages for genetic analysis and breeding work. They are used to mate with cultivated or wild diploid species. The diploid hybrid progenies are screened for superior performance of agronomic traits and, more importantly, for their ability to produce unreduced 2n gametes. The 2n gametes are formed due to the failure of either the first or the second division during meiosis. The first division restitution (FDR) and second division restitution (SDR) gametes have fundamentally different consequences on genetic segregation. The FDR gametes are capable of maintaining about 80% of the heterozygosity of a parent and thus are beneficial in maintaining heterosis in the progenies when it is used to cross with other tetraploid parents. Hybrid progenies from 4x x 2x matings are now produced in many breeding programs. Monohaploids (2n=x=12) of potato can also be obtained from the tetraploid varieteies. Two successive cycles of chromosome number reductions are involved. A tetraploid is reduced to a dihaploid. The dihaploid is further reduced to a monohaploid. Monohaploids are also obtained from diploid species and interspecific hybrids. Monohaploids are useful tools for critical genetic analysis of the diploid S. tuberosum species (Jacobsen and Ramanna, 1994). Monoploids provide the cytological evidence of the basic chromosome number (x=12) of the potato genome. Monohaploid progenies are used to screen out undesirable and lethal genes and identify beneficial mutants. Doubling chromosomes of monohaploids lead to the production homozygous diploids and tetraploids. The hemizygous condition of monohaploids also facilitate mapping of molecular markers due to the simple (1:1) segregation ratio. Both di- and monohaploids can be produced by two methods: parthenogenesis and androgenesis. Production of dihaploids by parthenogenesis is a well established procedure and used in potato breeding to generate parents for 4x x 2x crosses. Anther or microspare culture appears to be the preferred procedure to obtain a large number of monohaploids because the number of microspores far exceeds the number of ovules in an ovary (Jacobsen and Ramanna, 1994).