S E E D S I Z E

Euphytica 14 (1965): 143-152

IN T H E C U L T I V A T E D POTATOES

N. W. SIMMONDS John Innes Institute, Hertford, England

With two figures

Received 25 Nov. 1964

ABSTRACT

The three main groups of cultivated potatoes were studied (diploids (2n ---- 24), Andigena (2n = 48) and Tuberosum (2n ~ 48)). Seed weights were measured as mg per 100 seeds. In the tetraploids, but not in the diploids, there is a regression of seed weight on seed number per berry with b --~ -7 mg per 100 per 100 seed, implying nutritional competition between seeds. This effect accounts for very little variation between samples however. Tetraploid seeds are about 30 % larger than diploid seeds but there is much overlap between the two groups; genetic control of seed number appears to be predominantly bi-parental, of seed size predominantly maternal, but there are exceptions. Tuberosum seed is slightly larger than Andigena seed and the difference is attributable to the decline in fertility that occurred during the evolution of the former Group. There is no evidence of increase of seed size correlated with selection for plant vigour; experiments with sieved seed of different size grades showed that genetic differences independent of seed size overwhelmingly controlled plant vigour but that there was a transient maternal effect of seed size apparent in the young seedlings and disappearing later in life. General conclusions are: that seed size in the cultivated potatoes is determined by ploidy, by genetic factors (mostly maternal) and by maternal nutritional effects; and that it offers no correlations with plant characters that might be useful to the plant breeder.

INTRODUCTION

The north temperate potatoes (Tuberosum Group) evolved by selection from intro- duced South American tetraploid cultivars (Andigena Group) and the Andigena Group itself evolved, probably by autotetraploidy, from South American cultivated diploids. It is known that tetraploid seed is larger than diploid (SIMMONDS, 1963) but that there is much genetic variability within the two great groups of cultivars. An attempt is being made to recreate the Tuberosum Group (John Innes Institute, 1961, 1962; SIMMONDS, 1965) and, to this end, various methods of mass selection involving seed and seedling characters are being tried. The finding that seed size and seedling growth are correlated prompted this survey of the genetical/physiological relations of seed size in the cultivated potatoes.

MATERIAL AND METHODS

Plant Materials

Most of the materials used came from the South American cultivated potatoes. Some were diploid (2n = 24) and within them two cultivar Groups are distinguished: Stenotomum (abbreviated stn) and Phureja (phu) (DODOS and PAX~N, 1962). The

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rest were tetraploid (2n = 48) and are referred to the Andigena (adg) or to the Tubero- sum (tbr) Groups of cultivars.

Twelve lots of material (A-L) were examined, as follows: Diploid A - phu/stn crosses previously used in seed and tuber dormancy studies (SIMMONDS, 1964); 11 parents, 49 crosses, some replicates; glasshouse pollinations, 1961. B - phu/stn diallel cross previously used in seed dormancy studies; four phu, two stn parents, triplicated; glasshouse pollinations, 1962. C - Samples from Commonwealth Potato Collection; 37 crosses (10 phu, 27 stn) 35 different females by mostly different males; no replicates; glasshouse pollinations, 1963. D - Experiment comparing performance of diploids and tetraploids (John Innes Institute, 1963); 20 open-pollinated berries from 20 dif- ferent females; field, 1960. Tetraploid E - Samples of adg from Commonwealth Potato Collection; 60 females × 60 different males; no replicates; glasshouse pollina- tions, 1960-63. F - Crosses between adg breeding clones; 39 parents, 235 crosses; glasshouse pollinations, 1961-63; also two subsamples in duplicate for error and re- gression estimation. G - Mass selection experiment with adg (see text); 17 bulk seed samples from populations over five years of selection; field 1959-63. H - Experiment comparing diploids and tetraploids (see D above); 13 open-pollinated berries from 13 different females; field, 1960. K - Crosses between tbr breeding clones; five independent sets of pollinations covering a very wide range of parents; various partial replications; glasshouse, 1959/61/62/63.

Methods Most observations were based upon clean air-dried seeds extracted from mature

berries set by hand-pollinations in the glasshouse. Observations of immature samples showed that seed weight increased up to 5-6 weeks but not thereafter, the attainment of maximal size thus corresponding with the full establishment of dormancy (S~M- MONDS, 1963). A "mature berry" is therefore defined as six weeks or older. Seed samples, generally 100 or more and never less than 20, were weighed to the nearest milligram and results are given as mg per 100.

Among the products of glasshouse pollinations, there was some variation between years. Thus 13 crosses of cultivated diploids made in both 1961 and 1962 showed that years agreed well as to numbers of seeds per berry but not as to seed weight, which was significantly greater in 1962 than in 1961. Interactions of years with parents were small in both. On the other hand, four sets of Andigena pollinations made in the years 1960-1963 gave constant mean seed weights. However, even in the diploids, the years effect is far less than that of ploidy and it does not affect the comparisons made here.

RESULTS

General The results of estimations of seed numbers and weights of various samples of cul-

tivated potatoes are summarized in Fig. 1. There are four points to make. 1. In descending order of female fertility, the groups go; diploids, Andigena,

Tuberosum. It is known that the same order applies to numbers of berries set in the field (John Innes Institute, 1963) so it is clear that there has been a steady and marked reduction of female fertility during the evolution of the potatoes: indeed there has even been some conscious selection against fertility in Tuberosum breeding (FINEMAN, 1947).

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SEED SIZE IN THE C U L T I V A T E D POTATOES

90

4 7 0 " t ~ T u b e r o s u m ~ K " " ' " ' _ f i e ld . . . . . . . . . - ~ / ' ~ - ~ . . . . . . . . . . . . . . . . .

IG 5 0 ° ~ ] ~ - ~ A n d i g e n a - f i e l d + / i p l ° i d s ~B ~C - - -

- I D ~- d ip lo ids - f i e l d

E seeds pe r b e r r y 30 , , J , - , - , - , - , - , " , " , " ' " ' " '

2 0 6 0 100 140 180 2 2 0 2 6 0

FIG. 1. Seeds per berry and seed weights in the cultivated potatoes. Above and on right hand side - observations based on glasshouse berries plotted as a scatter diagram of means with lines indicating the 5 ~ fiducial limits of each sample; for reference letters see text (Plant Materials); the regression line (see text) has a slope - 7 mg per 100 seeds. Bottom left - weights of samples of field seed, horizontal arrows indicating means, vertical lines 5 ~ fiducial limits.

2. The possibility of correlation between numbers of seeds and average weight was investigated. For the diploids, no evidence of correlation was found. For the Andi-

gena Group, correlation was immediately evident from plotting and three estimates of it were obtained. The replicated subsample of I tem F (see above) gave r[26] = -0.754 and a regression of -7 .1 mg per 100 per 100 seeds. Second, the main population of Item F consisted of 235 crosses of 29 females by various males and the within-females items of the analysis gave r[205] = -0.422, b = -6.7 mg per 100 per 100 seeds. And, third, sparse and generous self pollination of three Andigena clones gave berries with a wide range of seed numbers for which the within-clones items gave r[18] = -0.695, b -- -10.0 mg per 100 per 100 seeds. The regression will be taken as -7 mg, a value with which the third estimate does not disagree (t = 1.2).

In the Andigena Group, therefore, a low seed yield is partly compensated by an increased seed weight and vice versa. In the Tuberosum Group, (item K), three samples gave no sign of correlation and one was weakly negative. This, and the distribution of means in Fig. 1, suggests that the correlation is indeed present in the Group, though weak. It may be that the regression coefficient falls at the lowest levels of seed yield, and this would not be unexpected. 3. The effect of ploidy is clear: Andigena seeds are about 3 0 ~ heavier than diploid

seeds and this is, no doubt, a direct effect of tetraploidy. Though populations of samples undoubtedly differ, there is much variation within each group and an overlap of size over much of the range (Fig. 2). Tuberosum seeds are slightly larger than those of Andigena but this is attributable to the fertility difference between the Groups (see above and Figs. 1 and 2). Glasshouse seed is heavier than field seed and this may be partly due to the inclusion of immature berries in field samples. Since viability tests showed that field samples were generally of high average maturity, however, this cannot be the whole explanation: though levels are lower than in the glasshouse, the

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N. W. SIMMONDS

contrast between diploids and tetraploids is again apparent (Fig. 1). Three Tuberosum samples lie towards the upper end of the Andigena range.

means diploid adg tbr

40-

u e [-J diploid

~" ~j~ ~ ~ l i ~ ,adg 20 ~ tbr

-

I I I I | I I I I I 2O 4O 6O 80 100 size classes FIG. 2. Distribution of seed sizes in the cultivated potatoes. Size classes 20-29, 30-39 mg per 100 etc.;

numbers of samples - diploids 97, adg 351, tbr 118.

4. The evidence in the last paragraph suggested that there had been no shift of seed weight during the evolution of the Tuberosum Group from Andigena, except for

that which could be accounted for by declining fertility. Further evidence on this point comes from experiments in which an attempt is being made to recreate the Tuberosum Group (John Innes Institute, 1961, 1962; SIMMONDS, 1965). After five years of selection the current populations are estimated to be about half-way to Tuberosum. Various bulk lots of field seed were taken each year and random samples were weighed; there was no evidence of differences between selection cycles in seed weight. The same con- clusion emerges from Fig. 1: lot E represents unselected Andigena clones, Lot F breeding clones after three or four generations of selection; no difference is apparent.

Genetic aspects

The results of a diallel cross (Item B above) among the cultivated diploids are sum- marized in Tables 1 and 2. The whole was triplicated but two plots were missing, due to incompatibility; missing values were therefore calculated and error has only 56 degrees of freedom.

For seed numbers, females accounted for a very large part of the variance, differences between Males corrected for Females being, indeed, non-significant; and the large Reciprocal Differences additive c.a. item in Table 2 reflects this. Though control was predominantly female, there were uninterpretable non-additive effects:

Item d.f. V

Females 5 200,370 (

Parents / Males corrected 5 6,870 t for females

Residual non-additive effects 17 13,370 Error 56 4,246

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SEED S IZ E IN T H E C U L T I V A T E D P O T A T O E S

TABLE 1. A DIALLEL CROSS IN THE CULTIVATED DIPLOID POTATOES

top - m e a n seeds per berry, b o t t o m - m e a n seed weight in m g per 100

Females Males

836 979 1776 2211 2749 1855

836 - 84 69 66 68 75 979 155 - 199 207 85 104

1776 254 283 - 273 135 217 2211 387 343 372 - 515 374 2749 86 122 244 254 - (145) 1855 420 244 265 272 (277) -

836 - 45 53 47 50 51 979 55 - 64 57 54 62

1776 50 57 - 58 51 60 2211 59 60 68 - 47 60 2749 59 61 57 54 - (60) 1855 53 50 61 44 (47) -

Leas t significant differences between any two entries ( 5 ~ ) : seed number , 106 seeds; weight, 9 rag. Bracketed entries are caJculated miss ing values.

TABLE 2. ANALYSIS OF DIALLEL CROSS IN THE CULTIVATED DIPLOIDS (CF. TABLE 1)

above, var iances; below, fitted additive cons tan ts

I t em d.f. Seed N u m b e r s Seed Weights

Reciprocal Sums Addit ive c.a. 5 97038 *** 147 *** Res idual 8 17016 *** 78 *

Reciprocal Differences Addit ive c.a. 5 110202 *** 318 *** Residual 9 10128 * 23 Error 56 4246 30

Paren t male female male female

836 + 1 2 - 145 - 1.2 - 6 . 2 979 - 19 - 74 + 0 . I +3 .3

1776 + 1 3 + 15 +5 .7 + 1 . 2 2211 + 3 2 + 185 - 2.5 + 3 . 2 2749 - 14 - 53 - 4 . 9 + 2 . l 1855 - 2 3 + 71 +2 .8 - 3 . 6

Significance of var iance rat ios shown thus : * 5 ~ , ** 1 700, *** 0.1 5/00

A n a l y s i s b y f i t t i n g m u l t i p l i c a t i v e c o n s t a n t s (GILBERT a n d JINKS, 1964) s h o w e d t h a t

o n e p a r e n t , C . P . C . 2 7 4 9 , i n t e r a c t e d m o r e s t r o n g l y t h a n a n y o f t h e o t h e r s a n d , i n d e e d ,

a c c o u n t e d f o r m u c h o f t h e n o n - a d d i t i v e v a r i a n c e . T h e p a r e n t s t h e r e f o r e d i d n o t

b e h a v e u n i f o r m l y .

F o r s e e d w e i g h t s , c o n t r o l w a s e q u a l l y b i p a r e n t a l a n d a d d i t i v e e f f e c t s a c c o u n t e d f o r

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nearly all the variance. Perhaps surprisingly, reciprocal differences were important: three clones (979, 2211 and 2749) tended to transmit larger seeds as females than as males, and three (836, 1776 and 1855) tended to the reverse. It is clear therefore that the male parent influenced seed weight though not seed number.

No other complete diallel is available but there are data extensive enough for ana- lysis by the method of fractional replication which fits additive constants to parents in incomplete diallel crosses (example of use in BROWN, 1960). Results (Table 3) show

TABLE 3. ANALYSES OF SEED YIELD AND WEIGHT DATA IN POTATOES variances, with degrees of freedom in brackets

Diallel Analysis

Data additive c.a. remainder

Parents

females males corrected corrected for males for females

Seeds per berry Andigena 1961 (F) 741(14) 193(18) 942(7) 552(10) Andigena 1962/3 (F) 3586(26) 562(184) 2219(24) 2247(23) Tuberosum 1962 (K) 1107(8) 393(12) 814(7) 1016(5)

Seed weights Andigena 1961 (F) 537(14) 182(18) 878(7) 192(10) Andigena 1962/3 (F) 1208(26) 237(184) 1388(24) 409(23) Diploids 1961 (A) 250(10) 109(38) 415(10) 71(10)

Variances of seeds per berry divided by 10.

that additive combining ability accounted for a large part of the variance; that seeds per berry was biparentally determined but that seed weights were very largely female determined. This conclusion as to male and female contributions disagrees with the results of the diallel cross described above but is based upon a wider range of material and upon a much larger number of parents. The tentative conclusion is that seed number is predominantly biparental (but with some over-riding female influences) and that seed weight is predominantly female (but with some male influences in par- ticular combinations). That male effects on seed weight, at least among the tetraploids, are not negligible is suggested by comparison of the relevant male variances in Table 3 with an error variance of 32 (n = 52, weighted mean of three independent estimates of error from tetraploid data).

It was shown above that seed yields and weights are negatively correlated, certainly in Andigena, probably also in Tuberosum. Inspection suggested that the regression (-7 mg per 100 per 100 seeds) was too small to account for much of the variability in seed weight and this was confirmed by analysis of four sets of adjusted tetraploid data: the general means and variances between crosses were unaltered though error was, as expected, somewhat reduced. Thus there were large differences between females in seed weight independently of minor differences due to number of seeds per berry.

General conclusions are as follows. The finding that seed yields are predominantly biparental while seed weights are maternal implies that the former is largely controlled by zygote/endosperm survival, the latter by maternal nutritional factors rather than by

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the hybrid genotype, the only diploid maternal tissue in the potato seed being the thin testa. That maternal factors are important is also implied by the existence, in tetraploids, of a negative correlation between seed yMd and seed weight though the apparent absence of this relation in the diploids is a mystery. General experience with potatoes is that, where seed-yield is concerned, there are "good" and "bad" male and female parents, in agreement with the conclusion that seed yield is generally biparental. Predominantly maternal control of seed weight implies that selection at this level should be rather inefficient in terms of securing genetic advance in plant characters correlated with seed weight and experiments on this point are described below.

Seed size and seedling characters

The results of an experiment in which seeds of different size were compared as to the characters of the resulting seedlings are summarized in Table 4. Two lots of seeds

TABLE 4. SEED SIZE SELECTION AND SEEDLING CHARACTERS IN THE ANDIGENA POTATOES

Seed size Character Lot L.S.D. (5 ~)

smal l medium large

Height (cm) A 14.3 19.1 17.1 3.1, 4.3 B 9.0 16.7 16.6

Weight (g) A 1.53 2.03 3.12 0.91, 1.28 B 1.48 3.88 4.64

"Sturdiness" A 103 99 190 51, 72 (mg/cm) B 153 243 285

Lot A - unselected, Lot B - selected (see text). The three entries for lot B small seeds based on 10 observations, all others on 29 to 32 observations; in last column first entry refers to comparison of two means based on 30 observations, second to two means based on 30 and 10.

were used: A, random, unselected Andigena and B, Andigena derived from the same population as A but after two generations of mass selection for general vigour and adaptation to growing conditions in England. Mean seed weights were nearly identical (A 49, B 51 mg per 100). Seed bulks were sieved into three lots: small (passed 1 mm round-mesh sieve, 2 ~ of population), medium (held by 1 mm sieve, 80 ~o) and large (held by 11 mm sieve, 18 ~). Mean seed weights of the three lots were: small, 22 mg per 100; medium 47; large 65. Seedlings were grown in pots in the glasshouse for seven weeks and heights were measured weekly; differences were apparent very early. After seven weeks, tops were cut off at soil level and weighed. There was little branching so weight/final height is a reasonable measure of "sturdiness" (Table 4).

Seed size selection had an effect on all three characters, large seeds producing taller, heavier and sturdier seedlings. The two lots, A and B, agreed reasonably well in response to selection but differed in mean values: A, the unselected Andigena, were roughly equal to B in height (or possibly a little taller) but were generally lighter and less sturdy, a difference which reflects the two generations of selection for general vigour to which B had been subjected. Selection was therefore effective within lots but the large difference between A and B (with similar average seed sizes) shows that seed size can account for only a very small part of the variation in vigour.

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Seed size and mature plant characters

The results of an experiment in which the later performance of plants raised from sieved seed of different size grades was studied are shown in Table 5. The material used was Andigena selected for growth in England, comparable with but slightly more advanced than lot B in the previous section. Mean seed weights were: small, 29; medium, 50; large, 66 mg per 100 seeds. Plants were grown in a randomized block experiment in the field with four replicates of 25-plant plots. An early subsample of 5 plants per plot was taken in late July and the rest were harvested in early September. Early growth appeared to be better in the plants raised from the larger seeds but the difference was, in fact, non-significant for top weights (V.R. [2,6] = 2.00) though signi- ficant for tuber yield (V.R. [2,6] = 16.3). At the main harvest, at which theplants were still short of full maturity, no significant differences were found and the early seed size

TABLE 5. SEED SIZE SELECTION AND MATURE PLANT CHARACTERS IN THE ANDIGENA POTATOES

Seed Size Measurement Harvest

small medium large

Top wt., g early 272 296 353 late 768 958 923

Tuber numbers early 3.8 4.1 5.5 late 14.4 13.3 11.3

Tuber yield, g. early* 24 33 54 late 225 234 205

Tuber wt., g early 6 8 10 late 63 72 73

Top/tuber ratio early 11.3 9.0 6.5 late 3.4 4.1 4.5

* Differences significant at 5 ~ level, others non-significant. All data given on a per plant basis.

difference had disappeared or, at least, was too small to be detected. The effect of seed size on plant characters therefore appears to be a transient maternal nutritional one unrelated to intrinsic vigour or productivity. In this Andigena material, at least, nothing would be gained by selecting for it.

DISCUSSION

Four factors influence seed size in the cultivated potatoes: (1) ploidy, tetraploid seeds being larger than diploid; (2) maternal genetic effects, which account for most of the differences between seed lots within ploidies and widely transgress the effects of ploidy; (3) maternal nutritional effects which are revealed as differences between seed weights that are dependent upon numbers of seeds per berry but independent of genetic constitution; and (4) paternal effects which are sporadic in occurrence and generally small.

Tuberosum seeds are slightly heavier than those of Andigena and the difference is accounted for by decline of seed fertility (and concomitant increase in seed weight) in

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SEED SIZE IN THE CULTIVATED POTATOES

the evolution of the north temperate cultivars. There is no evidence that seed weight has increased as a correlate of selection for, for example, enhanced plant vigour. Experiments support this conclusion: seed weight selection in Andigena produced only a transient effect on seedling growth rate which declined by mid-growth and dis- appeared later. Such transient maternal effects though unimportant in potatoes (and useless to the potato breeder) are not negligible in other plants and studies of various north American grasses and legumes suggest that large-seededness may be agronomic- ally valuable in aiding establishment though not subsequent performance (HENSON and TAYMAN, 1961; KITTOCK and PATTERSON, 1962; KNEEBONE and CREMER, 1955; ROGLER, 1954). In Lolium (THOMAS, 1963), there is "appreciable additive genetic variation between different populations together with a large maternal effect" and, apparently, a rather more persistent correlation between plant characters (leaf size and tillering) and seed size within strains than in the examples just cited. A maternal effect on seed size has also been found in rice (CHANDRARATNA and SAKAI, 1960) but not in Lima beans (ALLARD, 1956). In barley, KAUEMANN and MCFADDEN (1960) found that non-genetic variation in seed size profoundly affected the performance and competitive ability of the plants, producing effects on agronomic characters roughly comparable in magnitude with the genetic effects that the plant breeder seeks to recognize. So far as the information goes, therefore, it looks as though variation in seed size often has a strong transient maternal component which may be of some use to the agronomist but is merely a nuisance to the plant breeder. But, considering the economic importance of seed size in many crops, it is surprising how little solid in- formation there is about its genetic control and relation with seed number.

ACKNOWLEDGEMENT

Thanks are due to MR. N. E. GILBERT for statistical advice.

REFERENCES

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4. DODDS, K. S. and PAXMAN, G. T., 1962. The genetic system of cultivated diploid potatoes. Evolution N. Y. 16: 154-167.

5. FtNEMAN, Z. N., 1947. Elimination and retention of pollen sterility in potato improvement. J. agric. Res. 75: 134-145.

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12. R.OGLER, G. A., 1954. Seed size and seedling vigour in crested wheatgrass. Agron. J. 46: 216-220. 13. SIMMONDS, N. W., 1963. Experiments on the germination of potato seeds. Europ. Pot. J. 6: 45-60,

69-76. 14. SIMMONDS, N. W., 1964. The genetics of seed and tuber dormancy in the cultivated potatoes.

Heredity 19: 489-504. 15. SIMMONDS, N. W., 1965. Studies of the tetraploid potatoes. III. Progress in the experimental recre-

ation of the Tuberosum Group. J. Linn. Soc. Hot. (in the press). 16. THOMAS, R. L., 1963. Seed weight. Welsh P1. Hr. Stn. annu. Rep. 1962: 15-16.

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