776
Fertility of somatic hybrids Solanum
commersonii (2x, 1EBN) (+) S. tuberosum haploid
(2x, 2EBN) in intra- and inter-EBN crosses1
D. Carputo, P. Garreffa, M. Mazzei, L. Monti, and T. Cardi
Abstract: Solanum commersonii (+) S. tuberosum hybrids with different endosperm balance numbers (EBN) and ploidy
were used in various crossing schemes with 4EBN S. tuberosum cultivars and a 3EBN somatic hybrid to test their
behavior in intra- and inter-EBN crosses and to derive a BC1 population for potato breeding. The somatic hybrids
included 12 tetraploids (2n = 48, 3EBN), 18 hypotetraploids (2n = 43–47, 3EBN), 2 hexaploids (2n = 72, 4 or 5EBN),
and 5 hypohexaploids (2n = 63–70, 4 or 5EBN). The results confirmed that in the potato, EBN is more important than
actual ploidy in determining the success or failure of crosses. Indeed, crosses between some hypohexaploid hybrids and
S. tuberosum cultivars gave the highest number of viable seeds per berry, and seed viability was similar to that of
intra-EBN crosses between varieties. Inter-EBN crosses demonstrated that exceptions to the 2:1 maternal–paternal EBN
ratio in the hybrid endosperm can sometimes be tolerated. However, an excess in maternal EBN dosage was better
tolerated than an excess in paternal EBN.
Key words: EBN, ploidy, potato, Solanum commersonii, somatic hybrids.
Résumé : Des hybrides Solanum commersonii (+) S. tuberosum présentant des niveaux EBN (<<Endosperm Balance
Number >>) et de ploïdie différents ont été croisés avec des cultivars du S. tuberosum à 4EBN et un hybride
somatique à 3EBN afin d’observer leur comportement dans de tels croisements intra- et inter-EBN et afin de dériver
des populations BC1 pour l’amélioration génétique. Les hybrides somatiques incluaient 12 tétraploïdes (2n = 48,
3EBN), 18 hypotétraploïdes (2n = 43–47, 3EBN), 2 hexaploïdes (2n = 72, 4 ou 5EBN) et 5 hypohexaploïdes
(2n = 63–70, 4 ou 5 EBN). Les résultats confirment que, chez la pomme de terre, l’EBN est plus déterminant que le
niveau de ploïdie en ce qui a trait à la réussite ou l’échec de croisements. De fait, des croisements entre certains
hybrides hypohexaploïdes et des cultivars du S. tuberosum ont produit le plus grand nombre de graines viables par baie
et la viabilité des graines était semblable à celle de graines provenant de croisements intra-EBN impliquant différents
cultivars. Les croisements inter-EBN ont démontré que des exceptions à la régle du 2 : 1 au niveau du ratio entre
l’EBN maternel et paternel chez l’albumen hybride peuvent parfois être tolérées. Cependant, un excés de dose EBN
maternel était mieux toléré qu’un excès paternel.
Mots clés : EBN, ploïdie, pomme de terre, Solanum commersonii, hybrides somatiques.
[Traduit par la Rédaction]
Carputo et al.
781
Postzygotic barriers to interspecific hybridization can
hamper the exploitation of some diploid (2n = 24) Solanum
species which possess useful traits for potato breeding.
These barriers frequently result in endosperm breakdown
and thus embryo degeneration. To explain the behaviour of
interspecific crosses in Solanum, each species is assigned a
Corresponding Editor: J.P. Gustafson.
Received November 17, 1997. Accepted April 21, 1998.
D. Carputo, P. Garreffa, M. Mazzei, and L. Monti.
Department of Agronomy and Plant Genetics, University
of Naples, Via Università 100, 80055 Portici, Italy.
T. Cardi.2 CNR–IMOF, Research Institute for Vegetable
and Ornamental Plant Breeding, Via Università 133,
80055 Portici, Italy.
1
2
CNR–IMOF contribution no. 164.
Author to whom correspondence should be addressed
(e-mail: [email protected]).
Genome 41: 776–781 (1998)
hypothetical value, the endosperm balance number (EBN),
which is given according to its crossability with species with
known EBN standards (Johnston et al. 1980). The EBN has
been regarded as the “effective ploidy” of Solanum species,
and it is not necessarily equivalent to the chromosome
ploidy (Ortiz and Ehlenfeldt 1992). The EBN must be in a
2:1 maternal–paternal ratio in the hybrid endosperm for normal endosperm and embryo development. This implies that,
when 2n gametes are not produced, a balanced EBN ratio is
obtained only if the male and the female parents have the
same EBN. The EBN represents a strong isolating mechanism in Solanum. Diploid species with EBN = 1 are sexually
isolated, because crosses with S. tuberosum (tbr) haploids
(2n = 24, 2EBN) and other diploid 2EBN species fail owing
to an unbalanced EBN ratio in the hybrid endosperm.
Studies on the genetic control of EBN in the potato suggest
that EBN is controlled by three unlinked genes operating in
a threshold-like system (Ehlenfeldt and Hanneman 1988;
Johnston and Hanneman 1996).
Different breeding strategies have been employed to overcome inter-EBN barriers. Sexual hybrids between either dip© 1998 NRC Canada
Carputo et al.
loid 1EBN species or tetraploid 2EBN species and tbr have
been obtained through in vivo EBN manipulations
(Ehlenfeldt and Hanneman 1984; Bamberg et al. 1994;
Carputo et al. 1997). Embryo culture was used by Singsit
and Hanneman (1991) to circumvent the EBN barriers of
1EBN S. chancayense (2n = 24), and by Watanabe et al.
(1992, 1995) to attain germplasm enhancement with 2EBN
S. acaule (2n = 48), and with diploid 1EBN non-tuber
bearing species S. brevidens, S. etuberosum, and
S. fernandezianum.
Somatic hybridization by protoplast fusion may also provide an important tool to overcome inter-EBN crossing barriers and to broaden the genetic basis of the cultivated
potato. It allows doubling of ploidy level and the combining
of intact parental genomes, without meiotic segregation. It
also permits mitochondrial genome recombination and new
assortments of cellular organelles (Wenzel 1994).
Interspecific hybrids with various EBN and ploidies have
been obtained between tbr and a number of 1EBN diploid
species, including S. brevidens (Barsby et al. 1984; Austin et
al. 1988), S. circaeifolium (Mattheij et al. 1992),
S. commersonii (Cardi et al. 1993a), and S. pinnatisectum
(Menke et al. 1996). However, the potential of somatic hybridization in plant breeding can be fully exploited only
when meiotic recombination between parental chromosomes
occurs, and when the hybrids are fertile and can be crossed
with tbr. This enables the selection of desirable alleles
through backcrosses.
S. commersonii (cmm) is a noteworthy species for potato
breeding owing to its high resistance to low temperatures as
well as its cold acclimation capacity. It is also resistant to
various biotic stresses and has a high specific gravity of tubers. Somatic hybrids of cmm (+) tbr displayed introgression
of genes for frost tolerance and cold acclimation capacity
from the wild species, and intermediate phenotype for a
number of morphological traits. Cytological analysis demonstrated that they were either tetraploid to hypotetraploid with
EBN = 3, or hexaploid to hypohexaploid with EBN = 4 or 5
(Cardi et al. 1993b). These hybrids not only represent potentially useful material for potato breeding, but also provide
unique genotypes that help to understand the role of EBN in
seed development and to interpret results from novel EBN
ratios. In fact, the availability of genotypes with EBNs of 3,
4, or 5 allows the EBN model to be tested under different
maternal–paternal EBN ratios.
In the present study we report on the crossability of these
somatic hybrids with several 4EBN cultivars and a rare male
fertile 3EBN cmm (+) tbr hybrid. The objectives were to derive a BC1 population for breeding purposes, and to test the
behavior of the somatic hybrids in intra- and inter-EBN
crosses.
Somatic hybrids were obtained by Cardi et al. (1993a) from
electrofusion of mesophyll protoplasts of cmm (PI 243503) and the
tbr haploid SVP11. A total of 37 somatic hybrids with good flowering characteristics were selected for crossability studies. They included 12 tetraploids (ploidy group A; 2n = 48, 3EBN), 18
hypotetraploids (ploidy group B; 2n = 43–47, 3EBN), 2 hexaploids
(ploidy group C; 2n = 72, 4 or 5EBN), and 5 hypohexaploids
(ploidy group D; 2n = 63–70, 4 or 5EBN). Tetraploid and
777
hypotetraploid hybrids have two haploid genomes from cmm and
two from tbr, thus their EBN is 3. Hexaploid and hypohexaploid
genotypes have an EBN of 4 or 5 depending on their genomic
composition. If they have four cmm genomes and two tbr genomes
their EBN should be 4, whereas if they have two genomes of cmm
and four of tbr their EBN should be 5. The tbr cultivars (4EBN)
LT-5, LT-7, AVRDC-1287.19, Tollocan, and Carmine were chosen for their good performance as male and female parents in
southern Italy.
All the somatic hybrids were female fertile, except for the
tetraploid SH9A, which was both male and female fertile and also
self-compatible (Cardi et al. 1993b). For this reason, the somatic
hybrids were crossed as female parents with tbr cultivars and with
SH9A. Reciprocal crosses with 4EBN tbr could be performed only
with the fully fertile 3EBN clone SH9A. All plants were grown under screenhouse conditions in Camigliatello Silano, Italy, in the
summers of 1990, 1991, 1992, and 1993, and crosses were generally repeated over these years. Fresh pollen, collected one to two
hours before pollination, was applied to stigmas after emasculation
of flowers. For each cross combination, 20–40 flowers were pollinated. Fruits were allowed to ripen for about 6–8 wk, and then
seeds were extracted. The seeds were viewed under a
stereomicroscope to separate those containing the embryo from the
aborted ones. Four fertility parameters were evaluated: fruit set, total number of seeds per berry (TSB), total number of viable seeds
per berry (VSB), and percent seed viability that was calculated as
(VSB / TSB) × 100.
The data obtained were subjected to analysis of variance
(ANOVA) using the “General Linear Model” procedure in Minitab
statistical software, release 10.5 Xtra (Minitab Inc., State College,
Penn., U.S.A.). The original data were checked for the homogeneity of error variances by Bartlett’s and Levene’s tests, and for the
normal distribution of experimental errors by KolmogorovSmirnov’s test (Steel and Torrie 1980; Snedecor and Cochran
1989). Since the results of these tests were significant, data were
transformed using log10 (x + 1) transformation. Transformed data
had errors with homogeneous variances and normal distributions;
hence, all statistical analyses were done on transformed data.
When treatment effects were significant in ANOVA, multiple comparisons of means were carried out using the Duncan’s multiple
range test (Steel and Torrie 1980).
Somatic hybrids × 4EBN tbr crosses
Results from inter-EBN crosses of somatic hybrids used
as females and 4EBN tbr used as male provided evidence of
a high variability among the somatic hybrids in terms of fertility parameters. As expected, TSB and VSB were much
lower than that obtained from tbr × tbr crosses. The results
pooled by ploidy levels of the somatic hybrids are reported
in Table 1. Significant and nonsignificant differences between years and between 4EBN tbr male parents, respectively, were observed. Thus, results of crosses are presented
separately for each year, but are pooled for the tbr parents.
The effect of the ploidy level of the female parent was significant (P < 0.01). The highest values of TSB were observed in group A (4x/3EBN) in 1992 (28.8 seeds per berry)
and in 1993 (60.9 seeds per berry). Values displayed by
groups B (<4x/3EBN), C (6x/4 or 5EBN) and D (<6x/4 or
5EBN) were much lower, ranging from 2.7 (group B in
1991) to 23.2 (group C in 1992). These results may be due
to the fact that genotypes of group A are tetraploid, and thus
are likely to produce a higher frequency of balanced functional gametes than genotypes of ploidy groups B, C, and D.
© 1998 NRC Canada
778
Genome Vol. 41, 1998
Table 1. (A) Percentage of berry set, total number of seeds per berry (TSB), number of viable seeds per berry
(VSB), and % seed viability in intra- or inter-EBN crosses of somatic hybrids grouped by ploidy level as females
and five tetraploid cultivars (tbr) as males. For comparisons, results from tbr × tbr crosses are also reported.
Ploidy group
A (4x)
Parental EBN
(maternal:paternal)
3:4
B (<4x)
C (6x)
3:4
4 or 5:4
D (<6x)
4 or 5:4
tbr × tbr
4:4
Year
Berry set (%)
TSBa
1991
1992
1993
1991
1992
1993
1991
1992
1993
1992
1993
—
55
64
—
33
62
—
52
52
44
25
6.8
28.8
60.9
2.7
23.2
14.7
11.3
6.6
14.9
255.7
273.3
VSBa
cd
ab
a
d
cd
bcd
bcd
d
abc
0.7
1.7
4.4
0.9
0.0
0.7
7.0
1.6
10.9
235.4
230.3
Seed viability (%)b
d
bcd
b
cd
d
cd
abc
cd
a
10
6
7
33
0
5
62
24
73
92
84
Table 1. (B) Mean squares values from ANOVA for TSB and VSB.
Source
df
TSB
VSB
Ploidy group
tbr male parent
Year
Error
3
4
2
68
1.58**
0.13 ns
3.02**
0.23
0.67**
0.01 ns
0.98**
0.07
Note: —, Data not available; **, Significant at P < 0.01; ns, Not significant.
a
Log10 (x + 1) transformations of original data were used for statistical analyses. For each trait, means followed by the same letter
were not significantly different at P = 0.05 on the basis of Duncan’s multiple range test. Coefficients of variation for TSB and VSB
were 58.7% and 84.7%, respectively.
b
(VSB / TSB × 100).
There was a significant effect of ploidy group on VSB.
Hypohexaploids (group D) gave the highest values of VSB
in 1991 and 1993 (7.0 and 10.9 in 1991 and 1993, respectively). Though there was a high variability among the group
D hypohexaploids, SH5A and SH8A (2n = 70 and 65, respectively) consistently surpassed all the other genotypes in
the group (not shown). Ploidy group D showed a seed viability comparable to that of intra-EBN tbr × tbr crosses. In
1992, the seed viability of group D was lower (24%), but it
did surpass the VSB obtained by the other groups in the
same year.
(Hypo)hexaploid cmm (+) tbr have either two genomes
of cmm (1EBN) and four of tbr (4EBN), or four genomes
of cmm (2EBN) and two of tbr (2EBN). Thus they may have
an EBN of 5 or 4, depending on their genomic composition.
Genotypes with 4EBN produced gametes, which gave a
compatible 2:1 EBN ratio in the hybrid endosperm following crosses with tbr (4EBN), whereas in genotypes with
5EBN this ratio had a slight excess in female dosage (5:2).
The variability in VSB found in hypohexaploid genotypes
may be explained assuming that the genotypes which performed well are 4EBN, while those which performed poorly
have an EBN of 5. An EBN of 5 may also account for the
low VSB of hexaploid genotypes. Molecular analyses on
these hybrids, however, are necessary to confirm their
genomic composition and EBN. Alternatively, it is possible
that the hypohexaploid genotypes with high VSB values
somehow tolerate maternal–paternal EBN imbalances. Exceptions to the 2:1 EBN ratio have already been reported
(Ehlenfeldt and Helgeson 1987; Ehlenfeldt and Hanneman
1988; Louwes et al. 1992). Genetic studies of the EBN system have indicated that an EBN ratio which is slightly
higher than 2:1 can sometimes be tolerated in inter-EBN
crosses (Ehlenfeldt and Hanneman 1988). Recently,
Johnston and Hanneman (1995, 1996) hypothesized that
seed development in inter-EBN crosses may be due to incomplete penetrance of the EBN ratio requirement, to unusual fertilization events, or endomitosis of the polar nuclei.
Masuelli and Camadro (1997) interpreted the unexpected
success of inter-EBN crosses being due to segregation for
EBN. The positive performance of some hypohexaploid somatic hybrids may also indicate that these genotypes lack
the chromosome(s) involved in the control of the EBN system, allowing the unbalanced maternal–paternal EBN ratio
in the endosperm to become balanced (from 5:2 to 2:1).
Ehlenfeldt and Helgeson (1987) and Rokka et al. (1994)
analyzed the fertility of 3EBN tetraploid and 4 to 5EBN
hexaploid S. brevidens (+) tbr somatic hybrids. The former
authors reported that despite a more abnormal meiosis and
lower percentage of stainable pollen of hexaploids compared
to tetraploids, hexaploid hybrids displayed a relatively high
female fertility after crosses with cultivars Kathadin (on average 15.6 seeds per berry) and Norland (on average 28.6
seeds per berry). Conversely, hypohexaploid hybrids produced by Rokka et al. (1994) were female sterile. However,
two of them displayed a high male fertility after crosses with
4EBN tbr (on average 55.1 seeds per berry). Jacobsen et al.
(1993) also produced hexaploid and tetraploid S. brevidens
(+) tbr hybrids. They found that ovule rescue and in vitro
germination were more successful in hexaploid × tbr combi© 1998 NRC Canada
Carputo et al.
779
Table 2. (A) Percentage of berry set, total number of seeds per berry (TSB), number of viable seeds per berry
(VSB), and % seed viability in reciprocal interEBN crosses of tetraploid somatic hybrid SH9A and five tetraploid
cultivars (tbr). For comparison, results from tbr × tbr crosses are also reported.
Female parent
Male parent
Parental EBN
(maternal:paternal)
Berry set (%)
TSBa
VSBa
Seed viability (%)b
LT-5
SH9A
Carmine
SH9A
AVRDC-1287.19
SH9A
Tollocan
SH9A
LT-7
SH9A
Average
tbr
SH9A
tbr
SH9A
LT-5
SH9A
Carmine
SH9A
AVRDC-1287.19
SH9A
Tollocan
SH9A
LT-7
4:3
3:4
4:3
3:4
4:3
3:4
4:3
3:4
4:3
3:4
—
—
64
45
34
27
11
70
7
62
151.3
3.3
81.1
57.4
39.5
16.3
161.2
94.6
97.0
24.7
42.3
1.0
45.9
5.1
26.9
0.4
114.2
6.4
46.5
5.6
28
30
57
9
68
2
71
7
48
23
SH9A
tbr
tbr
4:3
3:4
4:4
29
51
45
106.0
39.3
273.0
55.2
3.7
230.3
52
9
84
Table 2. (B) Mean squares values from ANOVA for TSB and VSB.
Source
df
TSB
VSB
Cross direction
Error
1
8
0.83 ns
0.17
3.12**
0.08
Note: —, Data not available; **, Significant at P < 0.01; ns, Not significant.
a
Log10 (x + 1) transformations of original data were used for statistical analyses. Coefficients of variation for TSB and VSB were
24.1% and 25.1%, respectively.
b
(VSB / TSB × 100).
nations. Indeed, out of 143 backcross hybrids produced, 114
(80%) had originated from hexaploid hybrids.
Crosses between 3EBN tetraploid and near-tetraploid somatic hybrids and 4EBN tbr produced only a few viable
seeds. A similar low fertility level of tetraploid somatic hybrids is reported by Ehlenfeldt and Helgeson (1987) and by
Rokka et al. (1994) in S. brevidens (+) tbr tetraploid hybrids.
These results were probably obtained because tetraploid somatic hybrids have an EBN of 3, and crosses with tbr result
in a 3:2 maternal–paternal EBN ratio in the hybrid endosperm.
Reciprocal crosses 3EBN SH9A × 4EBN tbr
The results of reciprocal inter-EBN crosses involving the
fully fertile hybrid 3EBN SH9A and five 4EBN tbr varieties
are reported in Table 2. The cross direction was highly significant (P < 0.01) for VSB and close to the limit of significance (P = 0.05) for TSB. The results obtained from these
inter-EBN reciprocal crosses between 3EBN SH9A and
4EBN tbr clearly demonstrate that viable seeds can be obtained in both cross directions, even though an excess in maternal EBN dosage is better tolerated than an excess in
paternal EBN. Indeed, in tbr × SH9A crosses TSB ranged
from 161.2 to 39.5 and VSB from 114.2 to 26.9. Conversely,
after the reciprocal crosses TSB ranged from 94.6 to 3.3 and
VSB from 6.4 to 0.4. On average, tbr × SH9A crosses produced 106.0 and 55.2, TSB and VSB, respectively. SH9A ×
tbr crosses produced 39.3 and only 3.7, TSB and VSB, respectively. Thus, seed viability was very much reduced
when SH9A was the female parent (9% vs. 52%).
To the best of the authors’ knowledge, this is the first report of successful inter-EBN reciprocal crosses. From the
analysis of published data, Hermsen (1994) concluded that
pollen tube inhibition usually prevents fertilization in the
failing direction when inter-EBN crosses are successful in
one direction only. In situ analyses of pollen tube development excluded stylar barriers between the somatic hybrid
SH9A and tbr, although they suggested a faster growth of
SH9A pollen tubes in tbr styles than in the opposite case
(Garreffa and Mazzei 1995).
Progenies from crosses between 3EBN SH9A and 4EBN
tbr are tetraploid, and can be directly used in backcross programs. The possibility of using SH9A both as a male and as
a female parent allowed the derivation of progenies with tbr
or recombinant cytoplasm, respectively. We found that even
though VSB from SH9A × tbr crosses was low due to endosperm abortion, an acceptable number of progenies could be
obtained and tested (Cardi et al. 1996). However, since TSB
was quite high (e.g., in SH9A × Tollocan), embryo rescue
could be employed to produce a large number of backcross
hybrids from these crosses. Jacobsen et al. (1993) successfully used embryo rescue to obtain backcross progenies
in inter-EBN crosses with somatic hybrids involving
S. brevidens.
Somatic hybrids × 3EBN SH9A crosses
The results of crosses between 3EBN and 4–5EBN somatic hybrids as female parents and 3EBN SH9A as
pollinator are given in Table 3. A high variability was found
among the female parents in terms of berry and seed set.
© 1998 NRC Canada
780
Genome Vol. 41, 1998
Table 3. (A) Percentage of berry set, total number of seeds per berry (TSB), number of viable seeds per berry
(VSB), and % seed viability in intra- or inter-EBN crosses of somatic hybrids grouped by ploidy level as females
and tetraploid somatic hybrid SH9A as male. For comparisons, results from SH9A either selfed or open pollinated
(OP) are also reported.
Ploidy
group
Male
parent
Parental EBN
(maternal:paternal)
Berry set (%)
TSBa
A (4x)
B (<4x)
C (6x)
D (<6x)
SH9A
SH9A
SH9A
SH9A
SH9A
SH9A
self
OP
3:3
3:3
4 or 5:3
4 or 5:3
3:3
3:
82
—
17
54
38
—
24.5
21.4
1.7
6.5
77.0
118.3
a
a
b
b
VSBa
Seed viability(%)b
18.6
12.9
0.5
0.8
64.5
98.5
76
60
29
12
84
83
a
b
c
c
Table 3. (B) Mean squares from ANOVA for TSB and VSB.
Source
df
TSB
VSB
Ploidy group
Year
Error
3
3
40
1.39**
0.02 ns
0.12
1.97**
0.03 ns
0.13
Note: —, Data not available; **, Significant at P < 0.01; ns, Not significant.
a
Log10 (x + 1) transformations of original data were used for statistical analyses. For each trait, means followed by the same letter
were not significantly different at P = 0.05 on the basis of Duncan’s multiple range test. Coefficients of variation for TSB and VSB
were 32.5% and 44.2%, respectively.
b
(VSB / TSB × 100).
The highest values of both TSB and VSB were shown by
SH9A both when selfed and when open pollinated. There
were no significant effects of the year on the fertility characteristics, hence results for different years were pooled. Significant differences were found between ploidy groups for
fertility characteristics. A and B ploidy groups (3EBN) were
more successful in crosses with SH9A than C and D ploidy
groups (4 or 5EBN). Indeed, TSB in A and B groups after
crosses with SH9A was 24.5 and 21.4, whereas VSB was
18.6 and 12.9, respectively. Conversely, C and D ploidy
groups gave much lower values: TSB and VSB were 1.7 and
6.5, and 0.5 and 0.8, respectively. It should be pointed out
that among genotypes of group D once again SH5A and
SH8A performed better than the others (data not shown).
These results suggest that the EBN for SH5A and SH8A
may be 4, while the other hypohexaploids may have a value
of 5. For this reason, crossing SH5A and SH8A with 3EBN
SH9A, should result in a less unbalanced maternal–paternal
EBN ratio (4:1.5) than the other hypohexaploids, which are
expected to result in a ratio of 5:1.5.
The availability of a female and male fertile 3EBN
tetraploid hybrid was extremely useful in these crosses because it permitted the verification of the EBN model through
intra-EBN crosses. Indeed, crosses between 3EBN tetraploid
or near tetraploid hybrids and 3EBN SH9A could be performed, and resulted in a seed viability as high as that of
intra-EBN tbr × tbr crosses. These results agree with those
expected under the EBN hypothesis, i.e., the parental EBNs
match and give a 2:1 maternal–paternal EBN ratio in the developing hybrid endosperm.
The results from inter and intra-EBN crosses with somatic
hybrids provide additional evidence that in the potato, EBN
is more important than actual ploidy in determining the success or failure of crosses. Indeed, crosses between genotypes
with different ploidy levels but the same EBN gave much
higher seed viability than those involving genotypes with the
same ploidy but different EBN. It has also been confirmed
that exceptions to the 2:1 maternal–paternal EBN ratio in the
hybrid endosperm can be tolerated in some instances.
Somatic hybrids between cmm and tbr were successfully
used in backcrosses with tbr, and viable seeds were obtained
from all the ploidy groups tested. The fertility of somatic
hybrids was a very important issue because often the main
bottleneck of somatic hybridization is the sterility of the genotypes produced. These results also demonstrate the usefulness of somatic hybridization to move the hitherto poorly
used cmm genes into a genetic background amenable to exploitation for potato breeding. The BC1 hybrids obtained
have been evaluated for a number of morphological and
physiological characteristics (Cardi et al. 1996). Research is
in progress to produce and characterize the BC2 progeny.
The authors thank Prof. S.J. Peloquin, University of
Wisconsin, Madison, Wis., U.S.A., for continuous support,
discussion, and critical reading of the original manuscript.
Austin, S., Lojkowska, E., Ehlenfeldt, M.K., Kelman, A., and
Helgeson, L.P. 1988. Fertile interspecific somatic hybrids of
Solanum: a novel source of resistance to Erwinia soft rot.
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