All the events of meiosis are under some form of genetic control and mutations which are defective in functions specific to meiosis have been identified in a variety of eukaryotes (Rees, 1961; Baker et al., 1976; Sears, 1976; Golubovskaya, 1979). Table 6.1, for example, summarizes the principal categories of mutations identified in plants. From such cases it has been inferred that the normal alleles of such mutations produce gene products that play crucial roles in regulating the meiotic activities of chromosomes in space and time. Like mutations generally, those which influence chromosome behaviour most commonly have deleterious consequences and lead to an impairment of the efficiency of meiosis. They have been most thoroughly characterized in the fruit fly Drosophila melanogaster and in the yeast Saccharomyces cerevisiae.

MEIOTIC MUTATIONS IN DROSOPHILA

A majority of mutations in Drosophila affect only one sex. This is not surprising when one recalls that SC formation and crossing-over is restricted to the female.

Male mutations

Microtubules, as we have seen, are dimers composed of equimolecular amounts of two 50000 dalton subunits, α-and β-tubulin. While there are other microtubule-associated proteins, the tubulins are the only ones that have been shown to play a role in chromosome movement since they are the principal functional components of the spindle microtubules. A specific β2-tubulin subunit has been identified which is expressed only at male meiosis in D. melanogaster and so functions in the production of the meiotic, but not the mitotic, spindle as well as in the differentiation of the axoneme of the male sperm (Kemphues et al., 1982).

INTRODUCTION

Soft rot (Erwinia carotovora ssp. atroseptica) and dry rot (Fusarium sulphureum) are serious storage diseases which must be considered in potato breeding. Resistance of potato cultivars and breeders' selections to both pathogens has been investigated. So far, resistant cultivars have not been developed. Differences within Solanum tuberosum are slight and are restricted primarily to grades of susceptibility.

MATERIALS AND METHODS

Soft rot: Tubers of 269 cultivars and breeders' selection were evaluated during 1983. These were divided into five groups according to origin. There were 42 cultivars, 116 S. tuberosum (tbr) breeders' selections, 66 hybrids of tbr x S. tuberosum ssp. andigena (adg), 12 hybrids of cv. Butte x S.microdontum (mcd) and 33 hybrids of tbr haploids x S. phureja (phu) or S. stenotomum (stn). A second test in 1983 and two tests in 1984 were conducted with a limited number of clones, including some which were very resistant.

Erwinia carotovora ssn. atroseptica (Eca) was isolated on Stewart's pectate medium from potato stems with typical blackleg symptoms. Dilution of Eca was made with sterile distilled water to obtain 5 × 106 colony forming units/ml. Ten ml of inoculum were injected at a depth of 2cm into each wound. The amount of tuber rot was calculated as the volume of decayed tissue (cm3) using the formula of Sorensen & Sparks (1980) for determination of the volume of bruises.

The virus programme, which was initiated in 1978, has given priority to breeding for resistance to the economically important potato viruses Y (PVY) and leafroll (PLRV). Sources of resistance to PVY were initially obtained from the Scottish Crop Research Institute, the Netherlands and Germany. Many of the accessions possess gene Ny from Solanum demissum which provides comprehensive hypersensitive resistance. Some have excellent PLRV resistance, and also field immunity to potato wart disease (Synchytrium endobioticum). More recently the Solanum stoloniferum gene Ry for extreme resistance, from sources such as the varieties Corine and Pirola, has been introduced into the programme, and we have also acquired accessions identified as PVY resistant by the International Potato Center (CIP). The development of lines duplex for Ny or Ry is in progress.

Breeding for resistance to infection with PLRV has utilized those PVY-resistant parents which also have PLRV resistance, and more recently accessions from CIP. Since PLRV resistance is less effective under severe infection pressure we are investigating the possibility that the gene for lethal hypersensitivity to PLRV present in varieties such as Monza, when incorporated into clones already possessing resistance to infection, will reduce incidence of the virus in the field. Tetraploid hybrids derived from S. brevidens will also be investigated as sources of PLRV resistance.

Many useful parental lines have been identified and potentially high-yielding clones possessing high resistance to PVY and PLRV are at all stages of selection.

INTRODUCTION

The commercial success of a new variety depends upon a number of factors, including field and storage characteristics and its suitability for a particular market or use. Information on some of these aspects is obtained by National List trials and the more comprehensive Recommended List trials of the National Institute of Agricultural Botany (NIAB) (Richardson, this volume). However, detailed information on the bulk harvesting and storage properties and the consumer acceptability of a new variety is not usually available until it is grown on a commercial scale. This may be several years after initial acceptance on to the National List and to bridge this gap the Potato Marketing Board (PMB) carries out a series of commercial scale trials and surveys.

TRIALS

Trials are grown from seed of common origin as part of the UK collaborative trial system. New varieties and their controls are grown according to local practice on collaborating farms in large plots (0.2 ha) for 2 years. Three sites are used for first and second early varieties and five for maincrop varieties. After harvest, produce of second early and maincrop varieties is transported to Sutton Bridge Experimental Station, where samples are taken for damage and quality assessments. Approximately 6 tonnes per variety and site are stored in 1-tonne pallet boxes at 7°C for between 4 and 5 months. CIPC is used for sprout control.

Damage levels, disease incidence and ware out-turns are assessed before and after storage.

INTRODUCTION

Potatoes differ from other arable crops grown in temperate countries in a number of ways which influence the conduct and logistics of breeding programmes and compound the difficulties of variety assessment. These differences can be summarized as follows.

1. The potato crop is propagated vegetatively from tubers and varieties exist as clones with the exception of crops grown from true potato seed (TPS), mainly in the tropics and subtropics. In temperate regions, however, the consequences of clonal methods include the establishment of a diversity of schemes for seed tuber multiplication, maintenance and certification of healthy stocks and of the importance given to breeding for resistance, especially to virus diseases.

2. There are more characters of economic importance in potatoes than in any other temperate arable crop. This means that the chances of selecting a variety with good performance for all the characters of importance are very small. All varieties are, therefore, compromises and perform well or less well for the range of important characters. This has contributed to the relatively slow progress made in potato breeding and partly explains why the replacement rate of old varieties by new ones is less rapid in potatoes than in other arable crops.

3. Because of the autotetraploid nature of potatoes, the F1 population raised from intercrossing two tetraploid parents is highly heterozygous. Progeny testing as a means of evaluating parental performance is laborious and has seldom been applied routinely to breeding programmes.

In the German Democratic Republic (GDR) the potato is used as a high quality food for human consumption, as raw material in industry, and as fodder. Table 1 gives a survey of the development of the potato crop area and per capita consumption from 1970 to 1984. Potatoes are produced on specialized farms with an average potato crop area of 400 ha each. They are grown predominantly on sandy to loamy soil sometimes containing a large number of stones in the topsoil. Potatoes for specific purposes are grown in different areas of the GDR. Breeding of new varieties and clonal selection, as well as multiplication of high quality potatoes (Grades S, SE and E), are concentrated in the north where there is a low degree of virus infection. Blite and certified potatoes are multiplied in the central and southern areas of the country. Ware potatoes are produced in all regions. The consumer prefers a ware potato which becomes mealy to firm-fleshed on boiling and which is suitable for processing; special varieties for processing are not required. In view of this production system and the consumer requirements, potato breeding has the following main aims (Kleinhempel et al. 1983), some of which are dealt with in more detail in this paper:

1. 1) High yields in all maturity groups

2. 2) High proportion of marketable tubers

3. 3) Suitability for ware, starch or processing

INTRODUCTION

In West Germany potato breeding is carried out by private breeders. Nearly 20 breeding stations and breeders' co-operatives are concerned with the breeding of new varieties and clonal selection. In addition to this, four public institutes are working on breeding research to supply the private breeders with basic material. Separate research projects are also handled by university institutes.

The work of the potato breeders is documented in the national list of potato varieties published annually. In the 1985 edition 127 varieties are included. Five of these are for export use only and are not given detailed character descriptions. One hundred and fourteen varieties originate from German breeders; the remainder mainly come from Holland.

The multitude of breeders and varieties makes it impossible to define a uniform potato breeding strategy for West Germany or to give a complete survey of all the differences between the selection systems. In this paper only the main objectives and techniques are presented.

BREEDING OBJECTIVES

In the last 5 years, 42% of the total potato crop (7 million tonnes) has been used as ware potatoes, 29% in the processing industry and 18% for starch production, distillation and fodder (Graf & Menz 1984). Therefore all the breeding stations are producing new ware and starch varieties with intensive selection for processing quality.

INTRODUCTION

Recurrent selection and mass intercrossing are used to enhance resistance in domestic potatoes to potato virus Y, potato leafroll virus, Verticillium wilt, Columbia root knot nematode, and deep-pitted scab. Lines resistant to one of these diseases or pests are intercrossed to enhance that single attribute. Resultant true potato seed (TPS) is bulked and sown in a field nursery where severe selection pressure will identify increased resistance to that disease or pest. Clones from selected TPS plants are retested in the same nursery and also screened in other nurseries and performance trials to select for other attributes. Surplus pollen from intercrossing within each parental group is combined with pollen from the other four groups and used to intercross all five. It is also used to pollinate a parental group of breeding lines with superior horticultural attributes. TPS from these between-group mass intercrossings is sown in a field exposed to many diseases, pests and stresses to identify clones with multi-resistance. A number of multi-resistant breeding lines have been developed and are maintained in a disease and pest-free state by meristem culturing.

CHOOSING PARENTS FOR CROSSING GROUPS

To determine the best parents, we have tested known resistant germplasm from throughout the USA, Canada and elsewhere. In addition, several hundred advanced and early-generation selections are being obtained annually from US and Canadian breeding programmes.

The potato crop is of great economic importance in Poland. The area of land under potatoes is 2.2 million ha, which is about 15% of total arable land. This is due to the predominance of light soils (nearly 60% of total land) and suitable natural conditions for potato growing. Total annual potato production is 40 to 50 million tonnes. About 55% of total production is for fodder, mainly for pigs, and nearly 12% is used for domestic consumption. Potato is almost the only raw material available for alcohol and starch production. Considerable efforts are being made to increase exports and usage for industrial processing, i.e. alcohol, starch and especially dried food production.

During the next few years the area under potato is expected to decrease, but the average potato yield is expected to increase. The introduction of new, more valuable varieties is one of the main factors leading to the increase of yield.

The breeding of new potato varieties in Poland is coordinated by the Institute for Potato Research at Bonin, which has four Experimental Stations. Another five Plant Breeding Stations belong to the Association of Plant Breeding and Seed Production Enterprises. Preliminary trials are carried out at five or six centres over 3 years before official trials.

Official variety assessment has a long tradition in Poland. Since 1969, it has been the responsibility of the Research Centre for Cultivar Testing (COBORU) at Slupia Wielka.

INTRODUCTION

Until very recently the contribution of the diploid edible potatoes of South America to breeding in the Northern Hemisphere has been limited to a few, unimproved genotypes. To examine the potential of this material, a substantial and widely based sample should be acclimatized to higher latitudes, extensively screened for fungal and viral disease resistances, and suitable breeding strategies devised (bearing in mind its diploid nature) for combining it with a range of modern Solanum tuberosum ssp. tuberosum (Tuberosum) parents. Any general advantages which could reasonably be expected from the new material, such as more rapid attainment of particular breeding goals at the diploid level, the presence of unexploited additive variation for yield components, and of heterotic effects in hybrids should be utilized as fully as possible. This paper describes briefly a programme for the use of S. phureja (Phureja) germplasm at Pentlandfield, based upon these requirements.

METHODS

Work on the improvement and subsequent utilization of diploid potatoes began in 1967 with a mass-selection scheme, which was continued until 1979 (Carroll 1982). In 1968 crossing was started to produce first generation hybrids with dihaploids of Tuberosum. Further crossing and selection took place amongst the hybrid material, and a pedigree scheme for Phureja was initiated, using individual selections from the mass population. Direct crosses between Phureja and Tuberosum cultivars began on a small scale in 1973 using the “diplandroid” system to produce tetraploid offspring. Several hundred pollinations of 2x males with 4x females produced hardly any progeny based on “diplogynoids” (Sudheer (Carroll) 1977).

INTRODUCTION

In our country potato breeding is done by private breeders, with the exception of the Bayerische Landesanstalt für Bodenkultur und Pflanzenbau, Freising. Institutes like Max-Planck-Institut für Zuchtungsforschung in Cologne evaluate species from South America with resistance to diseases or other special qualities, and release their adapted clones to the breeders. The assessment of potato varieties is done by the Bundessortenamt at Hannover in cooperation with many other institutions. Like a patent office, the Bundessortenamt grants protection rights to the breeder for a new variety – Plant Breeders' Rights. This is based on the Variety Protection Act which was revised in 1985.

This Seed Act confirms that seed of agricultural species cannot be offered for sale unless the variety concerned is on the registered list of the Bundessortenamt. In order to be registered the variety must be distinct from other varieties, sufficiently uniform and stable, of value for cultivation and use and suitably named for registration. A variety is considered to be of value for cultivation and use if, in comparison with similar registered varieties, there is a clear improvement in characters concerned with crop cultivation, the utilization of harvested crops or any product obtained from such crops.

DISTINCTNESS, UNIFORMITY AND STABILITY

Assessment of DUS is based on sprout morphology under diffuse light and a field trial at one site with three replications for 2 to 3 years.

INTRODUCTION

In 1960 a “Catalogue of Cultivated Species and Varieties” was officially introduced and this provided a restricted list of varieties of certain species, the seed of which can be sold in France.

A variety is registered by the Ministry of Agriculture according to the advice of the “Comité Technique Permanent de la Sélection des Plantes Cultivées” (Permanent Technical Committee for Cultivated Plant Selection (CTPS)). This advice is based on the results of trials made by the “Groupement d'Etude et de Contrôle des Variétés et Semences” (Group for Study and Control of Varieties and Seeds (GEVES)) of INRA. Registration is for 10 years. It can be renewed for successive 5-year periods at the breeder's request and with the agreement of CTPS.

A breeder or his representative who wants a variety to be registered makes an application to CTPS. He has to pay registration fees for expenses for experimentation and has to provide seed equivalent in health status to basic seed, class E.

EXPERIMENTAL CONDITIONS

Official technical regulations include details of experimental procedures. The experimentation lasts at least 2 years. A variety can only be registered if it appears distinct from other varieties, stable and uniform after examination of its different characters. This examination is at present made by RIVRO, the Netherlands (see van der Woude, this volume), and was part of an agreement for cooperation between the two countries.