Use of mid-parent values and progeny tests to increase the efficiency of potato breeding for combined processing quality and disease and pest resistance.

A potato breeding strategy is presented which avoids the common but ineffective practice of intense early-generation visual selection between seedlings in a glasshouse and spaced plants at a seed site. Once pair crosses have been made, progeny tests are used to discard whole progenies before starting conventional within-progeny selection at the unreplicated small-plot stage. Clones are also visually selected from the best progenies for use as parents in the next cycle of crosses whilst they are multiplied to provide enough tubers for assessment of their yield and quality. Mid-parent values, as well as progeny tests, are then used to select between the resultant crosses. Material from other breeding programmes can be included in the parental assessments and used in the next cycle of crosses if superior. Finally, in seeking new cultivars, the number of clones on which to practise selection is increased by sowing more true seed of the best progenies, but without selection until the small-plot stage. Traits considered are resistance to late blight [ Phytophthora infestans (Mont.) de Bary] and to the white potato cyst nematode [ Globodera pallida (Stone)], fry colour and tuber yield and appearance, as visually assessed by breeders. The theoretical superiority of the strategy for seeking new cultivars lies in being able to practise between-cross selection for a number of economically important traits within 1 or 2 years of making crosses, something that is not possible on individuals as seedlings in the glasshouse or spaced plants at the seed site. This also means that full-sib family selection can be operated on a 3-year cycle, an improvement on current practice of clonal selection on what is often at least a nine-year cycle. New cultivars can be sought with more confidence from the best progenies in each cycle, and modern methods of rapid multiplication used to reduce the number of clonal generations required to find the best clones.

Construction of a genetic linkage map in tetraploid species using molecular markers.

This article presents methodology for the construction of a linkage map in an autotetraploid species, using either codominant or dominant molecular markers scored on two parents and their full-sib progeny. The steps of the analysis are as follows: identification of parental genotypes from the parental and offspring phenotypes; testing for independent segregation of markers; partition of markers into linkage groups using cluster analysis; maximum-likelihood estimation of the phase, recombination frequency, and LOD score for all pairs of markers in the same linkage group using the EM algorithm; ordering the markers and estimating distances between them; and reconstructing their linkage phases. The information from different marker configurations about the recombination frequency is examined and found to vary considerably, depending on the number of different alleles, the number of alleles shared by the parents, and the phase of the markers. The methods are applied to a simulated data set and to a small set of SSR and AFLP markers scored in a full-sib population of tetraploid potato.

Interval mapping of quantitative trait loci in autotetraploid species.

This article presents a method for QTL interval mapping in autotetraploid species for a full-sib family derived by crossing two parents. For each offspring, the marker information on each chromosome is used to identify possible configurations of chromosomes inherited from the two parents and the locations of crossovers on these chromosomes. A branch and bound algorithm is used to identify configurations with the minimum number of crossovers. From these configurations, the conditional probability of each possible QTL genotype for a series of positions along the chromosome can be estimated. An iterative weighted regression is then used to relate the trait values to the QTL genotype probabilities. A simulation study is performed to assess this approach and to investigate the effects of the proportion of codominant to dominant markers, the heritability, and the population size. We conclude that the method successfully locates QTL and estimates their parameters accurately, and we discuss different modes of action of the QTL that may be modeled.

Linkage analysis in tetraploid potato and association of markers with quantitative resistance to late blight (Phytophthora infestans).

We have constructed a partial linkage map in tetraploid potato which integrates simplex, duplex and double-simplex AFLP markers. The map consists of 231 maternal and 106 paternal markers with total map lengths of 990.9 cM and 484.6 cM. The longer of the two cumulative map lengths represents approximately 25% coverage of the genome. In tetraploids, much of the polymorphism between parental clones is masked by 'dosage' which significantly reduces the number of individual markers that can be scored in a population. Consequently, the major advantage of using AFLPs--their high multiplex ratio--is reduced to the point where the use of alternative multi-allelic marker types would be significantly more efficient. The segregation data and map information have been used in a QTL analysis of late blight resistance, and a multi-allelic locus at the proximal end of chromosome VIII has been identified which contributes significantly to the expression of resistance. No late blight resistance genes or QTLs have previously been mapped to this location.

Use of seedling progeny tests for genetical studies as part of a potato (Solanum tuberosum subsp. tuberosum) breeding programme.

A diallel set of crosses, including selfs and some reciprocal crosses, was made between 15 parents chosen for their male fertility from those included in a tetraploid potato (Solanum tuberosum subsp. tuberosum) breeding programme at the Scottish Crop Research Institute. Seedling progeny tests were used to evaluate the progenies for non-race-specific resistance to late blight (Phytophthora infestans) in both foliage and tubers, quantitative resistance to the white potato cystnematode (PCN) (Globodera pallida) and the commercial worth of their tubers as judged by breeders' visual preference. No reciprocal differences were found. Comparisons of the selfs and crosses revealed inbreeding depression for breeders' preference, which varied among the parents from negligible to severe, whilst there were also statistically significant differences for foliage and tuber blight, but not for PCN. When the selfs were omitted from the combining ability analyses, large differences in general combining ability (GCA) were found for all four traits, and smaller differences in specific combining ability for tuber blight and breeders' preference. The only statistically significant correlation between GCAs for different traits was a favourable one of r = 0.56 between foliage and tuber resistance to late blight. It was concluded that prospects were good for simultaneously improving all four traits by multitrait genotypic recurrent selection.

Producing commercially attractive, uniform true potato seed progenies: the influence of breeding scheme and parental genotype.

The production of attractive, uniform true potato seed (TPS) progenies was investigated. Four breeding schemes were compared: intercrossing tetraploid cultivars (cv x cv); doubled dihaploids x cultivars (ddh x cv); cultivars x diploid unreduced-gamete producers (cv x FDR) and doubled dihaploids x diploid unreduced-gamete producers (ddh x FDR). Fifty three progenies and five clones were grown in a glasshouse in a randomised complete block design with three replicates of 25 plants per progeny and clone. Each plant's tubers were counted and the colour, shape, quality of skin finish, flesh colour, and commencai attractiveness (which includes yield) recorded. The most uniform progenies were also selected by visual comparison with the clones. For mean attractiveness, differences (P < 0.001) between breeding schemes and between progenies within breeding schemes were detected. The cv x cv and cv x FDR progenies were more attractive than clonal controls. There were significant additive and non-additive effects for attractiveness in all breeding schemes except cv x FDR. There were between-progeny differences (P < 0.001) for uniformity for all characters. Progenies uniform for one character could be variable for other traits. Breeding schemes gave different levels of uniformity (P < 0.001) for all characters except shape and flesh colour, but none gave low levels of variation for all traits. Doubled-dihaploid parents increased the variation in progenies. There were uniformity differences (P < 0.001) between progenies within breeding schemes for all characters. Evidence of additive and nonadditive genetic variation for uniformity in all traits was detected. In each breeding scheme, parents with good general combining ability (GCA) for uniformity in several characters were identified. Visually selected uniform progenies had parents with good GCAs for uniformity in a range of traits and high specific combining abilities (SCAs) for several traits. A desynaptic first-division restitution (FDR) clone and a male-sterile doubled-dihaploid clone had the best GCAs for tuber uniformity in TPS progenies. Achieving multitrait uniformity in TPS is problematic but may be aided by the selection of parents with GCAs for uniformity coupled with progeny testing to allow for non-additive effects.

Resistance to potato leaf roll virus multiplication in potato is under major gene control.

The concentration of potato leaf roll virus (PLRV), as measured by a quantitative enzyme-linked immunosorbent assay, in the foliage of potato plants (Solanum tuberosum) of cv 'Maris Piper' with secondary infection was 2900 ng/g leaf, whereas in clones G7445(1) and G7032(5) it was 180 ng/g leaf and 120 ng/g leaf, respectively. To examine the genetic control of resistance to PLRV multiplication, reciprocal crosses were made between the susceptible cultivar 'Maris Piper' and the two resistant clones, and the three parents were selfed. Seedling progenies of these families were grown to generate tubers of individual genotypes (clones). Clonally propagated plants were graft-inoculated, and their daughter tubers were collected and used to grow plants with secondary infection in which PLRV concentration was estimated. The expression of resistance to PLRV multiplication had a bimodal distribution in progenies from crosses between 'Maris Piper' and either resistant clone, and also in progeny from selfing the resistant parents, with genotypes segregating into high and low virus titre groups. Only the progeny obtained from selfing 'Maris Piper' did not segregate, all genotypes being susceptible to PLRV multiplication. The pattern of segregation obtained from these progenies fits more closely with the genetical hypothesis that resistance to PLRV multiplication is controlled by two unlinked dominant complementary genes, both of which are required for resistance, than with the simpler hypothesis that resistance is conferred by a single dominant gene, as published previously.

Tolbutamide increases hypothalamic serotonin activity in the rat.

Sulfonylureas are potent hypoglycemic agents; however, their mechanism of action remains incompletely understood. Recent data indicate that hypothalamic norepinephrine (NE) plays a major role in mediating the central neural regulation of blood glucose. We therefore examined whether the sulfonylurea tolbutamide might lower serum glucose via an effect on hypothalamic NE neuronal activity, and compared the effects with those of 2-deoxyglucose-induced neuroglycopenia and of chronic insulin administration. Serum glucose levels fell and serum insulin levels rose 10, 20, and 30 min after acute tolbutamide injection. Serum glucose concentrations were reduced after chronic tolbutamide administered in drinking water, but serum insulin did not change. Hypothalamic NE neuronal activity was increased 10 min after tolbutamide administration, but not at the later times, nor during chronic tolbutamide administration. However, consistent with a rise in serotonin (5-HT) neuronal activity, hypothalamic 5-hydroxyindoleacetic acid (5-HIAA) and the 5-HIAA/5-HT ratio rose 30 min after acute tolbutamide and during chronic tolbutamide administration. This rise was not due to neuroglycopenia per se, since hypothalamic NE neuronal activity was increased and hypothalamic 5-HT neuronal activity was reduced after 2-deoxyglucose-induced neuroglycopenia. Furthermore, the effect of chronic tolbutamide contrasted with that of chronic insulin administration where hypothalamic NE neuronal activity was increased, while hypothalamic 5-HT neuronal activity was unchanged. We conclude that tolbutamide does not lower serum glucose via a direct effect on hypothalamic NE neuronal activity; however, we note that tolbutamide specifically increases hypothalamic 5-HT neuronal activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid bidirectional effects of insulin on hypothalamic noradrenergic and serotoninergic neuronal activity in the rat: role in glucose homeostasis.

Glucose release from the liver is mediated by hypothalamic norepinephrine (NE) neuronal activity, but glucose itself (or a metabolite of it) exerts negative feedback effects on central NE activity. The aim of the present study was to investigate a possible role for insulin in these central glucose homeostatic mechanisms. Computerized gas chromatography-mass spectrometry was used to assess the neuronal activities of hypothalamic NE and serotonin [5-hydroxytryptamine (5-HT)] in rats after acute insulin (2 U/kg) administration. Medial basal hypothalamic NE neuronal activity was assessed by the ratio of 3,4-dihydroxyphenylethyleneglycol to NE. The ratio was suppressed (P less than 0.005) 10 min after insulin administration but rose again to be significantly higher (P less than 0.05) than in saline controls by 30 min and at 45 min post insulin was highly significantly elevated. Hypothalamic 5-HT neuronal activity was assessed by the ratio of 5-hydroxyindoleacetic acid to 5-HT and showed effects opposite to those on NE and was elevated (P less than 0.0005) 10 min post insulin. Significant changes in serum corticosterone and GH levels also occurred after insulin administration, and the changes in these two hormones were positively associated with the changes in hypothalamic neuronal activities of NE and 5-HT, respectively. Serum glucagon levels were found to be significantly elevated in association with the secondary rise in hypothalamic NE activity but did not fall in the 10 min postinsulin phase thus indicating stimulation of pancreatic glucagon release by central NE neuronal pathways. The hypothalamic NE and 5-HT neural responses to a bolus dose of insulin were unaffected by feeding or fasting. These results and evidence that brain glucose utilization is reduced in the immediate postinsulin period suggest that the rapid effects of insulin on hypothalamic NE and 5-HT neuronal activities is a direct one not mediated by stimulation of brain glucose uptake.

Dopamine-beta-hydroxylase inhibition acutely stimulates rats hypothalamic noradrenaline and dopamine neuronal activity as assessed from metabolic ratios and circulating glucose and ACTH responses.

Because central noradrenaline neuronal activity is tonically inhibited by noradrenaline (NA) itself via an action at prejunctional alpha 2-adrenoceptors, it was hypothesised that the blockade of central NA synthesis following acute dopamine-beta -hydroxylase (DBH) inhibition might primarily deplete prejunctional NA levels and result in an increase in central NA neuronal activity through reduced NA autoinhibition. This hypothesis was tested in the rat following the acute administration of the DBH inhibitors diethyldithiocarbamate (DDC) and cysteamine (CSH). Computerised gas chromatography/mass spectrometry was used to precisely measure the hypothalamic levels of NA and dopamine (DA) together with those of their primary neuronal metabolites dihydroxyphenylethyleneglycol (DHPG) and dihydroxyphenylacetic acid (DOPAC), respectively. Both DDC (at 4 h) and CSH (at 30 min.) caused approximately a 50% reduction of hypothalamic NA concentrations. However this was associated with marked and highly significant increases in hypothalamic DHPG levels (by 50-100%) and in the hypothalamic ratio DHPG/NA. Also, when measured after CSH, the hypothalamic levels of the DHPG metabolite 3-methoxy-4-hydroxyphenylethyleneglycol were highly significantly increased. Consistent with increased DA neuronal activity, both DBH inhibitors raised DA and DOPAC levels and also the ratio DOPAC/DA in the hypothalami of treated rats and markedly suppressed serum prolactin levels (all p less than 0.01). The rise in hypothalamic concentrations of DHPG indicates that an increase in hypothalamic NA neuronal activity occurs following DBH inhibition. Significant elevations of blood glucose, corticosterone and ACTH were also observed after DBH inhibition. As we have previously demonstrated that increased central NA activity is associated with elevations of blood glucose, corticosterone and ACTH, these data provide further evidence for a functional increase in central NA activity caused by acute DBH inhibition. It is proposed that the increase in hypothalamic NA activity after DBH inhibition results from a primary depletion of the prejunctional alpha 2-active autoregulatory pool of NA.

The central vs. peripheral effects of clonidine on ACTH, corticosterone and glucose release.

The role of central vs. peripheral actions of clonidine was investigated in the rat following the separate and combined administration of clonidine and 2-deoxy-D-glucose (2-DG). Clonidine or 2-DG alone stimulated serum glucose and corticosterone but hypothalamic noradrenaline neuronal activity and ACTH release were stimulated by 2-DG only. The stimulation of noradrenaline neuronal activity and ACTH by 2-DG were totally blocked by clonidine. It was concluded that the increases in serum glucose and corticosterone after clonidine administration were not mediated by central effects but that alpha 2-adrenoceptor agonism by clonidine was responsible for inhibition of hypothalamic noradrenaline neuronal activity and pituitary ACTH release.

Relationships between brain noradrenergic activity and blood glucose.

Glucose is the principal energy substrate for the brain and studies have shown that the brain is able to increase glucose availability in the face of glucose starvation (neuroglycopaenia). The mechanisms, believed to be hypothalamic, that may be involved in a brain/blood glucose control system have not yet been identified. We have used novel techniques for assessing brain monoamine neuronal activity to investigate its relationship to blood glucose concentrations in the rat. We describe here two important relationships which emerge from these studies. One is that activation of hypothalamic noradrenaline (NA) activity following stress is associated with concurrent increases in plasma glucose concentrations. This relationship is linear and independent of the adrenal or pituitary glands. The second is an inverse relationship between plasma glucose concentration and hypothalamic NA neuronal activity--high blood glucose levels significantly inhibited the hypothalamic NA activity responses to stress, alpha 2-adrenergic blockade and adrenalectomy. Thus glucose (or a metabolite of it) seems to provide a negative feedback signal sensed by hypothalamic NA neuronal systems which, in turn, appear to stimulate liver glucose output by a neural mechanism.

Computer simulation of family selection schemes suitable for kale (Brassica oleracea L.), involving half-sib, full-sib and selfed families.

Three recurrent selection schemes suitable for kale (Brassica oleracea L.), involving half-sib (HS), full-sib (FS) and selfed (S) families, were compared by computer simulation. All combinations of 6, 12 and 24 families selected, out of 120 and 240 assessed, were investigated for a range of genetical models. Selection was simulated for 20 generations from an initial allele frequency of 0.05 and for 16 generations from an initial frequency of 0.20. With an initial frequency of 0.05 there was a serious loss of desired alleles ranging from 0.31 out of 20 for the HS scheme with 24 out of 240 families selected to 9.19 for the S scheme with 6 out of 120 families selected. It was concluded that if as many as 20 cultivars were included in the initial population the selection scheme should be chosen to minimise the loss. With an initial frequency of 0.20 there were no losses with 12 and 24 families selected in the HS and FS schemes respectively, and the highest loss was 2.88 for the S scheme with 6 out of 120 families selected. It was concluded that if as few as five cultivars were included in the initial population a compromise between the initial response to selection and the loss of desired alleles should be sought. Selecting 6, 12 and 24 families for the HS, FS and S schemes respectively, resulted in average relative responses per generation of 2.28, 2.74 and 2.76, respectively for the first five generations, and losses of 0.22, 0.13 and 0.35, respectively after 16 generations. Practical considerations favour the FS scheme over the S scheme.

Hypothalamic monoamine control of stress-induced adrenocorticotropin release in the rat.

Despite evidence that ACTH release after stress is under excitatory hypothalamic control, a stimulatory role for any of the monoamine neurotransmitters is yet to be clearly demonstrated. In the present investigation computerized gas chromatography/mass spectrometry was used to assess the neuronal activities of hypothalamic dopamine, norepinephrine (NE), and serotonin (5-HT) in rats after stress-induced ACTH release. Medial basal hypothalamic NE neuronal activity as assessed by the ratio of 3,4-dihydroxyphenylethyleneglycol (DHPG) to NE. (DHPG/NE) was elevated (P less than 0.0005) within 2 min after a 3-min cold water swim stress. Ether stress also caused a marked elevation in NE activity (P less than 0.0025). A highly significant positive correlation between the ratio of hypothalamic DHPG/NE and serum corticosterone was found over a large population of normal and stressed rats. Consistent with this relationship between hypothalamic NE neuronal activity and ACTH release being a causal one were the findings that 1) adrenalectomized rats exhibited markedly elevated hypothalamic DHPG/NE ratio and serum ACTH (both P less than 0.0005) together with serum corticosterone levels reduced to about 3% of control levels (P less than 0.0005), and 2) the administration to rats of the alpha-blocker yohimbine or the antianxiety agent diazepam resulted in significant changes in hypothalamic NE activity, together with parallel changes in ACTH secretion. In hypothyroid rats, which have elevated hypothalamic 5-HT activity, and in normal gentled rats, stress caused a significant reduction in hypothalamic 5-HT activity. High hypothalamic activity of dopamine or 5-HT in hypothyroid rats did not significantly affect basal ACTH levels nor prevent the responses to either cold water swim or ether stress, and both stresses resulted in elevated hypothalamic DHPG/NE, serum ACTH, and serum corticosterone (all P less than 0.005) in these animals. From these data it is concluded that NE is an excitatory hypothalamic monoamine for ACTH release in stress and that hypothalamic 5-HT activity is reduced after stress.

Effects of 6-methoxy-1,2,3,4-tetrahydro-beta-carboline and yohimbine on hypothalamic monoamine status and pituitary hormone release in the rat.

Shortly after administration of 6-methoxy-1,2,3,4-tetrahydro-beta-carboline (6-MeOTHBC) and yohimbine to normal or hypothyroid rats [the latter exhibiting chronically elevated levels of serotonin (5-HT) neuronal activity in the hypothalamus] there was a highly significant increase in hypothalamic noradrenaline (NA) activity and in ACTH release concomittant with a reduction in hypothalamic 5-HT activity (P less than 0.01) and in growth hormone (GH) (P less than 0.01) and in thyroid stimulating hormone (TSH) (P less than 0.01) release from the pituitary. Both compounds caused an increase in hypothalamic dopamine (DA) metabolism and in pituitary prolactin (PRL) release in normal rats (P less than 0.01) but only yohimbine exerted this action in hypothyroid rats. Lower doses of 6-MeOTHBC exerted a relatively specific effect in hypothyroid rats, reducing (P less than 0.01) 5-HT neuronal activity in parallel with pituitary TSH secretion (P less than 0.05). While gross effects of 6-MeOTHBC and yohimbine were similar with respect to their effects on NA and 5-HT status in the hypothalamus, there were quantitative differences. 6-MeOTHBC always caused a greater decrease in 5-HT turnover and a lesser increase in NA turnover than did yohimbine. On the basis of these studies we suggest that the effect of tetrahydro-beta-carboline-related alkaloids on pituitary hormone release may be due to their influence on hypothalamic monoamine status and the subsequent alteration of the hypothalamic-pituitary control system.

Different acute effects of the tyrosine hydroxylase inhibitors alpha-methyl-p-tyrosine and 3-iodo-L-tyrosine on hypothalamic noradrenaline activity and adrenocorticotrophin release in the rat.

Computerized gas chromatography-mass spectrometry techniques using selected ion monitoring and deuterated internal standards were used to assay simultaneously the medial basal hypothalamic concentrations of dopamine (DA) and noradrenaline (NA) and their major metabolites in individual rats 30 min after the administration of two different inhibitors of tyrosine hydroxylase, alpha-methyl-p-tyrosine (alpha-MT) and 3-iodo-L-tyrosine (MIT). Consistent with inhibition of DA synthesis, administration of both alpha-MT and MIT resulted in marked reductions (P less than 0.005) in the hypothalamic concentrations of DA and its metabolite homovanillic acid as well as in highly significant increases in prolactin secretion. alpha-MT administration, but not MIT, resulted in a highly significant decrease in NA concentration and a highly significant increase in the concentration of the NA metabolite 3,4-dihydroxyphenylethyleneglycol (DHPG). The hypothalamic ratio DHPG/NA was thus markedly increased (P less than 0.005) by alpha-MT indicating increased NA neuronal activity. alpha-MT administration also resulted in increased ACTH secretion (P less than 0.0005), an effect not observed following MIT. It is proposed that the effects on hypothalamic NA activity and ACTH secretion caused by alpha-MT are stress-mediated and unrelated to tyrosine hydroxylase inhibition. MIT is devoid of these effects but exhibits blockade activity, thus indicating it to be a preferable drug for the acute inhibition of tyrosine hydroxylase in neuroendocrine investigations.

Loss of hypothalamic dopaminergic control of prolactin secretion in the hyperprolactinaemic rat.

The possibility that chronic hyperprolactinaemia results in loss of the ability of hypothalamic dopamine activity to inhibit prolactin secretion was studied in rats. Two degrees of hyperprolactinaemia (moderate and gross) were induced in the animals following the chronic administration of two different doses of oestradiol valerate. In rats with high chronic serum prolactin concentrations (approximately 20 times normal) there was a profound increase in prolactin secretion following inhibition of brain dopamine (DA) synthesis by 3-iodo-L-tyrosine, indicating intact and highly active hypothalamic DA-inhibitory control of prolactin release. However, the degree of hypothalamic inhibition of prolactin release relative to normal controls was significantly reduced. In animals with grossly elevated chronic serum prolactin concentrations (approximately 100 times normal) a prolactin response to DA synthesis inhibition was absent despite a highly significant reduction in hypothalamic DA concentrations induced by 3-iodo-L-tyrosine. These observations show that chronic and gross hyperprolactinaemia in the rat results in loss of hypothalamic DA inhibitory control of prolactin secretion. The use of 3-iodo-L-tyrosine to block brain DA synthesis in these studies has provided significant new data relating to prolactin control in hyperprolactinaemic states and indicates that this compound could be a useful clinical tool in the study of human hyperprolactinaemia.