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XI. Hybridization and Chromosome Relationships

XI. Hybridization and Chromosome Relationships

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FIG. 15. Scanning electron micrographs of a pollen grain of pearl millet. (a) A spheroidal grain;

note a single germ pore (potus) with cmfa around it. (b) A portion of pollen grain magnified to show

the porus with costa (C)around i t . [(a) X2600; (b) x5400l



Natural hybridization between related species followed by natural polyploidy has

in fact given rise to our most important food, fiber, oilseed, and fodder crops.

The catalytic effect of hybridization on evolution lies primarily in enlarging the

size of the gene pool that would facilitate a favorable response of a population to

a changing environment (Grant, 1963; Stebbins, 1969, 1974).

The study of chromosome pairing in hybrids has facilitated genome analyses

and, thus, helped in the elucidation of phylogenetic relationships between different taxa. Meiotic pairing in hybrids, of course, depends upon the nature and

extent of differentiation among the parental genomes. Moreover, different types

of genetic control of chromosome pairing can complicate the pairing patterns in

intergeneric, interspecific, and even intervarietal hybrids between geographically

diverse ecotypes, especially of polyploid species (see Jauhar, 1975c, 1977b). In

view of these restrictions, de Wet and Harlan (1972) have questioned the value of

chromosome pairing data in interpreting phylogenetic affinities. However, I do

feel that, in spite of the aforesaid limitations, chiasmate pairing has provided and

will continue to provide useful information on the nature of ploidy of a species

and on phylogenetic affinities between species. Chiasmate pairing is a specific

process generally confined to chromosomes (or segments) of corresponding

genetic similarities. Homologous or homoeologous segments are probably able

to recognize each other based on congruence of pairing sites (recognition is

probably based on similarity in nucleotide sequences). Information on chromosome pairing in hybrids coupled with that in their amphiploids should therefore

give a realistic picture of genomic relationships of the parental species.

In the genus Pennisetum, some interspecific, intergeneric, and even intertribal

(Penniserum X Oryza) hybrids have been reported. The interspecific hybrids

involved mostly pearl millet as one of the parents, and have helped in the study of

chromosome relationships between pearl millet and several other species of



There are several semiwild, annual, diploid races in the section Penicillaria

(Table I) with which the cultivated pearl millet is interfertile and essentially

forms a single, composite reproductive unit. So the hybridization between pearl

millet and other annual penicillarias will, in effect, be intraspecific hybridization.

Pennisetum typhoides is a polymorphic species. Its polymorphism could be

due primarily to its hybridization with the taxa in the section Penicillaria. The

wild, annual relatives of pearl millet have been treated as separate species by

Stapf and Hubbard (1933, 1934) and Clayton (1972), although the latter

suggested their merger into single species with pearl millet. They are interfertile

with pearl millet and have contributed to its genetic enrichment. Such a process



of hybridization and intefflow of genes is facilitated by the protogynous nature of

the species.

Chromosome studies by Krishnaswamy (1951), Thevenin (1952), and Veyret

(1957) have shown that the annual relatives of pearl millet-viz., P . ancylochaete, P . cinereum, P . echinurus, P . gambiense, P . leonis, P . maiwa, P .

nigritarum, and P . pycnostachyum'-are diploids with 2n = 14 chromosomes

like the cultivated pearl millet (see Table I). Moreover, the fact that the chromosome morphology of several of these taxa was similar to one another and also to

that of pearl millet (Veyret, 1957) further indicated their affinity with pearl


P . violaceurn and P . fallax are two of the important wild, annual relatives of

pearl millet. Genetic studies by Bilquez and Lecomte (1969) and by Brunken

(1977) have shown that these taxa form fully fertile hybrids with pearl millet and,

hence, are not reproductively isolated from it. They must obviously have 2 n =

14 chromosomes in order to form fertile hybrids with pearl millet.

The chromosomal, hybridization and genetic studies showed conclusively that

the wild or semiwild annual relatives of pearl millet are not sufficiently isolated

from it to deserve specific ranks. Brunken (1977) has therefore merged all annual

penicillarias with pearl millet (Pennisefum typhoides), which he calls P .

americanum. For the sake of convenience, he has subdivided the species into

three subspecies: (1) subsp. americanum, which includes all the cultivated races

of pearl millet; (2) subsp. monodii, which encompasses all the wild and semiwild

races of pearl millet; and (3) subsp. stenostachyum, which is morphologically

intermediate between subspecies americanum and monodii, and includes all

mimetic weeds associated with the cultivation of pearl millet.




Several hybrids between pearl millet and other species of Pennisetum have

been made by different workers. In some cases amphidiploids have also been

produced by colchicine-induced chromosome doubling of the interspecific hybrids or by natural meiotic nonreduction.

1 . P . typhoides x P . purpureum Hybrids

Interspecific hybrids between P. typhoides ( 2 n = 14) and P . purpureum (2n

= 4x = 28) are the most widely studied in the genus Pennisetum and probably

also in the entire tribe Paniceae. These species of the section Penicillaria are

reported to cross in nature to produce spontaneous hybrids (Stapf and Hubbard,

'These are all infraspecific categories within the species P . ryphoides



1934). They have also been hybridized by numerous workers using either one of

them as the female parent. However, using pearl millet as the female parent has

several advantages: (1) its protogynous nature generally eliminates the need for

emasculation; (2) seed shattering is absent or minimal; and (3) it is easier to

identify hybrids at seedling stage.

Burton (1944), working in the United States, was the first to make interspecific hybrids, which were later produced in India (Krishnaswamy and Raman,

1949, 1953a,b; Krishnaswamy, 1951), South Africa (Gildenhuys, 1950; Gildenhuys and Brix, 1958, 1964), Pakistan (Khan and Rahman, 1963), Australia

(Pritchard, 1971; Muldoon and Pearson, 1977), Sri Lanka (Dhanapala et a l . ,

1972), Nigeria (Aken’Ova and Chheda, 1973), and other countries. The main

objective of hybridizing these species was to produce a high-quality, highyielding, perennial fodder plant that would inherit pearl millet’s forage quality,

nonshattering nature, and capacity to establish readily, and also have the perennial, aggressive nature and rust resistance of napier grass. In South Africa, the

main objective was to breed a large-seeded perennial for use in ley farming

(Gildenhuys, 1950).

The hybrids produced in different countries are generally high-yielding and

more acceptable as fodder plants than napier grass. They exhibit considerable

heterosis for both fodder yield and quality (Burton, 1944; Krishnaswamy and

Raman, 1949; Patil, 1963; Khan and Rahman, 1963; Hussain et al., 1968;

Pritchard, 1971; Aken’Ova et a f . , 1974; Gupta, 1974; Muldoon and Pearson,

1977). Powell and Burton (1966b) described a commercial method of producing

interspecific hybrids by using a male-sterile line of pearl millet (Tift 23A) as the

female parent; the hybrid thus produced was described as the highest-yielding

forage millet grown in the United States. However, the hybrids’ complete seed

sterility (they can be propagated only by vegetative means) has restricted their

adoption on a large scale. While these hybrids are widely grown in some countries, particularly in the Indian subcontinent, they are being grown in trials in

several other countries (see Muldoon and Pearson, 1979; see also Section

XI ,B,2 ,e) .

a. Gross Morphology. With respect to several vegetative characters such as

panicle morphology, internode length, leaf and ligule size, the hybrid is either

intermediate between the parents or more often approaches the purpureum parent. This is to be expected in view of the greater contribution of genetic material

from the purpureum parent. However, depending on the genotype of the parental

species used, there is a considerable variation in expression of heterosis for

different vegetative characters like height, stem thickness, tillering, and leaf size.

b. Chromosome Pairing. The hybrid is a triploid with 2n = 3x = 21

chromosomes (Fig. 16a), 7 contributed by diploid (2x) typhoides and 14 by

tetraploid (4x) purpureum. As discussed in Section I1,1, pearl millet has very

large chromosomes, larger than those known for any other species of Pen-


46 1

nisetum, except P . ramosum. Burton (1944) could identify the 7 large chromosomes of pearl millet ir. the interspecific hybrids, but he did not study chromosome pairing. Krishnaswamy (1951) and Krishnaswamy and Raman (1953a,

1954) studied chromosome pairing in the triploid hybrids. On the basis of the

formation of 711 + 7, in most of the cells and the absence of trivalents, these

authors concluded that the genome of typhoides was homologous to one of the

genomes of purpureum. Khan and Rahman (1963), Ramulu (1968), Sethi et al.

(1970), and Rangaswamy (1972) made essentially similar observations on

chromosome pairing in the hybrids and reached the same conclusions regarding

the genomic makeup of the parental species.

Raman (196% reviewing the earlier work of himself, Krishnaswamy, and

co-workers, designated the genomic constitution of P . ryphoides as AA, and that

of P . purpureum as A'A'BB. The formation of 711in the triploid hybrid (AA'B)

was attributed to synapsis between the A genome of typhoides and A' of purpureum. It was evident, however, that A and A' were not completely homologous.

c . Analysis of Inter- and Intragenomal Chromosome Pairing. In the hybrids

studied by Jauhar (1968), the easily recognizable size differences of the parental

chromosomes (Fig. 16a-c) permitted a detailed analysis of inter- and intragenomal chromosome pairing. A range of 0-9 bivalents was observed, the

mean per cell being 5.3. Whereas the majority of bivalents were formed between

A and A' genomes and, thus, were clearly heteromorphic (Fig. 16b,c), some

bivalents resulted from intragenomal pairing. Up to five heteromorphic AA'

bivalents were observed per cell, which suggested that the two genomes are

evolutionarily related, and that they probably arose from a common progenitor

with x = 5 chromosomes.

All three genomes-A, A', and B-showed

intragenomal (autosyndetic)

chromosome pairing. Such bivalents were almost homomorphic and hence easily

distinguishable from the heteromorphic, intergenomal (A-A') bivalents described above. The bivalents formed by the A genome chromosomes were the

largest (Fig. 16b), whereas those of A' and B genomes were, respectively,

intermediate and the smallest in size. The intragenomal pairing appeared to be

limited to a maximum of two bivalents, further suggesting x = 5 as the phyletically basic number from which, probably, the genomes with x = 7 were subsequently derived. Thus, it was inferred that x = 5 is the original basic number of

the genus Pennisetum and that P . typhoides is a secondarily balanced species

(Jauhar, 1968). Later studies by Jauhar (1970b), Minocha and M . Singh (1971b),

and Minocha and A. Singh (1971a) provided corroborative evidence favoring

these conclusions. It may be pointed out that as early as 1951, Krishnaswamy

had suggested that intragenomal pairing occurred within the B genome.

On the basis of pachytene pairing in the hybrid, Pantulu (1967b) reported that

the chromosomes 1-5 of typhoides were homologous with chromosomes 1-5 of



Flc. 16. Meiotic stages in the Penniserurn ryphoides X P . purpureum ( 2 n = 3 x = 21). Note

clear size differences of parental chromosomes. [Small arrow, wphoides univalents; medium arrows,

intragenomal bivalents within ryphoides complement; thick arrows, intergenomal bivalents formed

by A genome of typhoides and A ' of purpureurn.] (a) Metaphase I showing 21 chromosomes, 7 large

ryphoides chromosomes ( A genome) and 14 small purpureum chromosomes (A'B genomes). (b)

7, that comprise 1 heteromorphic intergenomal bivalent (thick m o w ) , 2 large

Metaphase I with 7,,




purpureum, and that chromosomes 6 and 7 of typhoides were homologous with

chromosomes 8 and 14 of purpureum, respectively. However, in view of the

markedly larger chromosomes of typhoides, a part of its chromosome should

remain unpaired with the corresponding purpureum chromosome. Consequently,

terminal forks, terminal unpaired regions, or intercalary loops should be seen at

pachytene. Pantulu (1967b) did not, however, report any such structures, as his

drawings of different bivalents show almost perfect pairing.

2 . P . typhoides x P . purpureum Amphidiploids

a . Chromosome Pairing and Fertility. Krishnaswamy and Raman (1949)

produced amphidiploids by treating the P. typhoides X P . purpureum hybrid

seedlings with 0.4% colchicine. Chromosome doubling largely restored the

fertility in the synthetic amphidiploids (AAA'A'BB; 2n = 6x = 42). During

meiosis they generally formed 2 I,,, multivalents being either absent or infrequent

(Krishnaswamy, 1951; Krishnaswamy and Raman, 1954; Ramulu, 1968, 1971;

Jauhar and Singh, 1969b; and Jauhar, unpublished results). Some univalents

observed at metaphase resulted from precocious separation of bivalents and

caused disjunctional abnormalities.

In view of the formation of 711 + 71 in the triploid hybrid (AA'B), some

quadrivalents or trivalents should be expected in the derived amphidiploids, but

they are observed rarely, if at all. This is probably due to preferential pairing

between the A-A, A'-A', and B-B genome chromosomes, resulting in 21,1.

Although the A and A' genomes are somewhat differentiated, the corresponding

chromosomes of these genomes can pair in the absence of their own homologous

partners. On chromosome doubling, however, bivalent formation is probably

brought about by strong preferential pairing. However, the possibility of some

sort of genetic control of pairing cannot be ruled out. The diploidizing genes in a

double dose probably bring about diploid-like (genetically enforced preferential)

pairing in the allohexapioid. [Such diploidizing genes that are effective only in a

double dose are known in polyploid species of Festuca (Jauhar, 1975a,c).] Thus,

the synthetic amphidiploid behaves meiotically like a typical allohexaploid.

b. Chromosomal Instability. Gildenhuys and Brix (1961) also produced an

amphidiploid by colchicine treatment of the cuttings of triploid hybrid. Although

largely fertile, the amphidiploid showed marked instability in somatic chromointrahaploid bivalents within typhoides complement (medium m o w ) , and 4 intragenomal bivalents

within A ' and B genomes; 2 large univalents belong to A genome and the remaining 5 univalents

belong to A' and B genomes. (c) Metaphase I with 6,, + 9,. Note clearly heteromorphic bivalents

(thick arrows) and distinct size differences among univalents. (d) A cell at anaphase with 22 chromosomes showing 2 typhoides and 5 purpureum chromosomes going to one pole, 3 fyphoides and 9

purpureum chromosomes going to other pole, and 3 Wphoides chromosomes lagging. The ryphoides

chromosomes are marked with m o w s . [(a-c) x ca. 1800; (d) x ca. 13501



some number within the plant; its number was in the range 2 n = 36-49, with 2 n

= 42 occurring most frequently (Gildenhuys and Brix, 1964). The backcross

progeny obtained from the cross 2x P. typhoides X 6x amphidiploid also

showed intraplant variation in chromosome number. Gildenhuys and Brix ( 1 964)

concluded that this intraplant numerical mosaicism was under genetic control and

that the genetic determinants expressed themselves only when present in a double

or higher dose. Thus, it appears that these genes were hemizygous ineffective.

Conversely, Krishnaswamy and Raman (1956) and Ramulu (1968, 1971) did

not report any intraplant or even interplant numerical mosaicism in the amphidiploids or their derivatives. Because of the formation of univalents, however, some

variation in chromosome numbers in the progenies of amphidiploids is inevitable

and may be detected if a large population is scored.

c . Phenotypic Manifestation of Different Genomes. The amphidiploids are

vigorous in growth and show gigantic features typical of several polyploids.

Generally, they have thicker stems, broader leaves, and larger panicles compared

to the parental triploids. They show greater morphological resemblance to P.

purpureum than to P . typhoides. This is expected in view of the greater genomic

contribution from the tetraploid purpureum. Contrarily, if it is considered that A

genome of typhoides and A‘ of purpureum are similar in genic content, then the

amphidiploids have, in effect, four A genomes and should resemble the

typhoides parent more closely. However, their greater resemblance to purpureum would show that either the A and A’ genomes are sufficiently differentiated and have different phenotypic expressions, or else it is more likely

that the B genome dominates the A as well as A‘ genomes and masks their

phenotypic manifestation.

That the B genome is indeed “dominant” is borne out by the studies of

Krishnaswamy and Raman (1949, 1953a, 1954, 1956), Raman and Krishnaswami (1961), Raman et al. (1963), and Raman and Nair (1964). All these

workers have demonstrated that even when the ratio of A to B genomes is altered

from 2 : 1 to 5 : 1, the phenotypic manifestation of the B genome is noticeably

greater than that of A genomes combined. In an amphiploid derivative with

AAAAA’B constitution (Raman and Krishnaswami, 1961), for example, there

was only one dose of B genome, but the characters of the wild parent, P.

purpureum, were still expressed. This indicated that one dose of B genome was

dominant (or perhaps epistatic) over five doses of the A genome. Studying the

morphology of selfed progenies of the amphidiploids by metroglyph analysis,

Ramulu and Ponnaiya (1967) and Ramulu (1971) also found a distinct skewedness towards P . purpureum in the expression of several morphological features.

d . Amphidiploid Derivatives. Synthetic amphidiploids have been successfully backcrossed to pearl millet to produce derivatives with different genomic

constitutions. Thus, Krishnaswamy and Raman (1956) found that hybrids could

be easily secured whichever way the cross was attempted, e.g., 2x x 4x, 2x x



6x, 4x x 2x,or 6x x 2x. All nine tetraploids produced from the cross 2n P.

typhoides x 6x amphidiploid had 2n = 28 chromosomes, showing that the

amphidiploids generally formed 2 1 -chromosome gametes or else such gametes

were at a competitive advantage over the aneuploid gametes. This is in sharp

contrast to the observations of Gildenhuys and Brix (1964), who reported that the

compatibility of the cross improved when subhaploid pollen of the hexaploid

fused with the haploid egg cell of P . typhoides.

Gildenhuys and Brix ( 1 964) also found a high degree of incompatibility when

amphidiploid was used as pollen parent in backcrosses to P . typhoides. Of the 31

offsprings thus obtained, only 4 had the expected number of 2n = 28 or less. The

remainder had 2n = 35 or thereabout, and hence arose from the unreduced egg

cells of P . typhoides. It appeared that the pollen from the hexaploid was more

compatible with diploid rather than haploid eggs of typhoides. Conversely, the

compatibility also seemed to be enhanced when the pollen from hexaploid contained less than the haploid set of chromosomes (Gildenhuys and Brix, 1964).

From the standpoint of plant breeding, however, this is not a welcome situation.

It was further inferred from these results as well as from the cross [2x P.

typhoides x 6x (P.typhoides x P . purpureum)] x 2x P . typhoides, that

incompatibility probably resided in the hybrid embryo itself and that its normal

development (e.g., the success of the cross) was not dependent on the normal

endosperm acting as a nurse tissue (Gildenhuys and Brix, 1964, 1965).

Chromosome pairing was studied in several other amphiploid derivatives, with

various genomic constitutions, obtained after a series of backcrossings of 6x

amphidiploid to 2x and 4x P. typhoides, followed by selfing and further crossing

of the derivatives (Raman and Krishnaswami, 1960, 1961; Raman et af., 1963;

Nair et al., 1964; Raman and Nair, 1964). As expected, these derivatives formed

different frequencies of multivalents, bivalents, and univalents, depending upon

their genomic composition. They were sterile by varying degrees.

From the studies on triploid hybrid (AA'B), the amphidiploids (AAA'A'BB),

autoallotetraploids (AAA'B) and numerous other amphiploid derivatives with

different genomic constitutions, the following conclusions can be made:

I . When present in duplicate, the genomes A, A', and B maintain their meiotic

integrity, forming mostly bivalents between homologous partners; this is probably due to genetically enforced preferential pairing.

2. When present in a single dose or in more than two doses, there is intergenomal pairing between A and A'; however, A-B and A'-B synapsis is

seemingly absent, showing thereby that A and A' are largely homologous to each

other, whereas they both are nonhomologous to the B genome.

3. There is some amount of intragenomal pairing probably limited to a

maximum of two bivalents within each of the three genomes.

These studies have confirmed the allotetraploid nature of P . purpureum. It is a



genomic allotetraploid (A’A’BB) with one genome largely homologous to the

typhoides (AA) genome at least in terms of its pairing behavior. The donor of B

genome may be one of the diploid members of the section Penicillaria, which may

exist somewhere in Africa or is probably extinct.

e . Selection of Superior Forage Hybrids. As described above, several

superior triploid hybrids between pearl millet and napier grass have been produced in different countries. However, they have not been adopted on a wide

scale primarily because they are completely sterile and can be propagated only by

vegetative means. From the point of view of easy distribution to farmers,

superior, fertile amphidiploid hybrids or derivatives would need to be developed,

so that seed can be supplied to farmers. Since the colchicine-induced amphidiploids are largely regular meiotically and their progenies show a wide range of

pollen and seed fertility, it may be possible to select fertile, superior forage

amphiploids which produce large amounts of good seed.

f. Possible Synthesis of Perennial Pearl Millet. From the point of view of

incorporating the desirable features of P . purpureum into P . vphoides, the

triploid plant obtained by Raman and Krishnaswami (1960) is interesting. This

plant was derived by crossing 2x typhoides (AA) with an autoallotetraploid

(AAA’B). The derived triploid had four nucleolar chromosomes, and the pattern

of pairing showed that it had mostly chromosomes of A and A’ genomes and

probably none of the B genome. It is interesting that most, if not all, B genome

chromosomes were selectively eliminated. Raman and Krishnaswami (1960)

suggested that from such triploid plants it might be possible to secure fertile

allodiploids with A and A’ genomes that would probably combine the desirable

features of ryphoides and purpureum.

It was observed earlier at the University of Hawaii Agricultural Experiment

Station (Anonymous, 1947) that the 34 F , hybrids, obtained by crossing a selfed

line of pearl millet (2n = 14) with an East African strain of napier grass (2n =

28), included plants with 2 n = 28,21, 18, 16, and 14. The occurrence of plants

with 2 n = 14 suggests the possibility of selecting the desired “allodiploid”

plants. In this way perhaps a perennial pearl millet can be produced. The selfed

pearl millet line used in the above cross was probably desynaptic and produced

imbalanced gametes and, hence, aneuploid hybrids. If a desynaptic purpureum

line can be used as a male parent and crossed with a diploid typhoides with

desirable features, different plants with a whole array of chromosome numbers

can be obtained to exercise selection for the desired 2 n = 14, meiotically regular, fertile plants.

In successive backcrosses of the 6x amphiploids to P . typhoides, Gildenhuys

and Brix (1969) observed a selective elimination of imbalanced gametes and

zygotes, particularly those with chromosomes of the B genome, and to a lesser

extent, those of the A‘ genome (of purpureum). After three generations of

backcrossing, all plants expressed the annual pearl millet habit. However, Gildenhuys and Brix (1969) could not combine the desirable features of typhoides



(viz., fertility, date of flowering, and grain size) with the perennial habit of

purpureurn. If further attempts are made using gamma-ray treatments also, it

may be possible to incorporate the desirable segments from purpureum into the

typhoides complement.

If a perennial pearl millet is indeed produced through the techniques described

above, it would be useful in the drought-stricken, semiarid regions of Africa,

tropical India, and other tropical and subtropical regions.

3 . P . typhoides x P . squamulatum

These species have been hybridized with a view to evolving a grass combining

the forage quality of pearl millet with the frost resistance and perenniality of P .

squamulatum (2n = 6x = 54). Patil et al. (1961) and the present author (Jauhar,

unpublished results) successfully obtained the hybrids. Taking advantage of the

protogynous nature of pearl millet, we dusted profusely the pollen of

squamulatum on a number of typhoides ears that had freshly emerged, receptive

stigmas. From the progeny, Patil et al. (1961) picked up a single hybrid that had

2n = 41 chromosomes. The hybrid obviously arose from the union of an unreduced 14-chromosome egg of pearl millet with a haploid 27-chromosome male

gamete of squamulatum. Morphologically, the hybrid resembled the squamulatum

parent very closely although it had the leafiness of pearl millet.

Patil et al. (1961) observed mostly 1611 9, in the hybrid. It appeared to the

authors that seven bivalents (which must have been much larger than the rest)

were formed by the typhoides complement, whereas the remaining nine bivalents

(the small ones) and nine univalents resulted from pairing within the

squamulatum complement. The small bivalents in the hybrid could have formed

as a result of intergenomal pairing between the three genomes of squamulatum

and some by intragenomal pairing.


4 . TrispeciJic Hybrids:

( P . typhoides x P . purpureum) x P . squamulatum

Several hybrids involving three distinct species have been made in grasses,

e.g., Lolium-Festuca complex (Jauhar, 1975b, 1976). In the genus Pennisetum,

there is one report of a trispecific hybrid obtained by crossing ( P . typhoides X P .

purpureum) hybrid with P . squamulatum (Rangaswamy and Ponnaiya, 1963).

The two hybrid plants thus obtained resembled the squamulutum parent in several features, including the panicle and spikelet characters. They also had penicillate anther tips, a typical character of typhoides (see Fig. 14) and purpureum.

Rangaswamy and Ponnaiya (1963) found that the hybrid had 2n = 48 chromosomes, which showed that it had arisen from an unreduced 21-chromosome egg

of the female parent (typhoides X purpureum hybrid) fertilized by the normal

27-chromosome male gamete from squamulatum. The formation of quadrivalents and pentavalents at meiosis suggested relationship of the genomes of the

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XI. Hybridization and Chromosome Relationships

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