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IV. Genetic Implications of Intergenomic Interactions

IV. Genetic Implications of Intergenomic Interactions

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heterosis as commonly observed in crop plants has recently been proposed

(Srivastava, 1981).




Division of chloroplasts in vitro, found by Ridley and Leech (1970), speaks in

favor of their genetic and biochemical autonomy and suggests that ct-DNA is

essential as a master molecule for chloroplast formation and development. Differentiation of plastids, however, depends on interaction of both ct-DNA and

nuclear genome (Leech, 1976). Photosynthesis is only one of the many biological functions of chloroplasts; others include protein synthesis, reproduction, and

nucleic acid replication. In higher plants chloroplast proteins may account for

more than 60% of the total cellular proteins. Approximately 50% of the proteins

in the chloroplast are soluble in aqueous solutions and are localized in the stroma.

In green leaves, the major soluble chloroplast protein is fraction 1 protein. Since

this leaf protein (referred to RBPCase, ribulose- 1,5-biphosphate carboxylase/

oxygenase) is found in all photosynthetic organisms, it is the most abundant

single protein species on earth. The genetic code for the transcription of the

functional molecule of RBPCase, as it is elaborated in greater detail in the

subsequent subsections, is contributed by both ct-DNA and nuclear DNA. The

remaining chloroplast protein is made up of insoluble or structural proteins that

constitute the thylakoid membranes. Photosystem I (PSI), photosystem I1 (PsII),

and light-harvesting chlorophyll proteins are found in this fraction. Two major

components in the chloroplast membrane of higher plants contain the bulk (75%)

of the energy-trapping chlorophyll molecules. These components are the PSI

chlorophyll protein and the light-harvesting chlorophyll a h protein (Thornberger,

1975). Genetic studies indicate that PSI chlorophyll protein is coded for by

ct-DNA (Hermann, 1971). This conclusion is based primarily on the genetic

analysis of a mutant en :alba-1 of Antirrhinum majus. This mutant does not

synthesize the PSI chlorophyll protein. Since this mutant is reported to be in the

plastome (Hermann, 19711, the site of coding information was construed to be in

ct-DNA. Machold and Aurich (1972) have provided evidence that this chlorophyll protein is synthesized on chloroplast ribosomes. This supports the notion

that ct-DNA may contain the genetic information for this chlorophyll protein.

In contrast nuclear DNA has the coding information for the light-harvesting

chlorophyl a h . This inference is deduced from the interspecific hybridization

studies of the Nicotiana species and supported by the antibiotic studies that

demonstrate that this protein is synthesized on the cytoplasmic ribosomes (Kung

et al., 1972).

Many reports provide further evidence that mutation of both nuclear and

chloroplast genes causes defects in PSI and PsII. Heber and Gottschalk (1964)



have described a nuclear mutant of Viciafubu with a block in PSI. Chloroplasts

of this mutant are incapable of NADP photoreduction. Bishop (1 972) induced a

nuclear mutation in Scenedesrnus obliquus with a block in the synthesis of

cytochrome f-552, which caused a suppression of cyclic phosphorylation. A

defect in PsII caused by mutation on one nuclear gene in the pea lethal mutant

and a block of electron transport chain (ETC) components between PSI and PsII

in the cotton nuclear mutant have been observed (Nasyrov, 1978). All these

results provide substantial evidence for the existence of an interaction between

nuclear-plasmon genetic systems ensuring the complete development of photosynthetically active thylakoid membranes. Mutations in genome-plastome genes

leading to defects in light-harvesting electron transport, photophosphorylation,

or NADP reduction will all result in impaired C 0 2 assimilation. The assemblage

of the photosynthetic unit of the thylakoid membrane therefore requires the

complementation of both plastome and nuclear genomes and of both chloroplast

and cytoplasmic ribosomes.

In order to understand the precise genetic control of chloroplast organization,

Alberts er ul. (1973) examined the time of appearance of the two major

chlorophyll proteins in young green beans (Phaseolus vulgaris). The lightharvesting chlorophyll a l b protein appeared first, followed by the PSI

chlorophyll protein. Thus the major chlorophyll a l b protein was first assembled

in the thylakoid membrane and the PSI chlorophyll protein was inserted later.

These results could be interpreted to mean that the biosynthesis of the lightharvesting chlorophyll a l b proteins serves as an initial step in the assemblage of

thylakoid membrane and that the nuclear genome probably regulates the development of the thylakoid membrane. By using the nuclear transplantation

technique in Acetabularia, such regulation was also demonstrated at the genomic

level (Kloppstech and Schweiger, 1974). Marked differences in the electrophoretic gel pattern of chloroplast membrane proteins from A . calyculus and A .

rnedirerrunea have been found. After isolated nuclei of A . rnediterrunea were

transplanted into basal parts of enucleated A . calyculus, the gel pattern of some

of the chloroplast membrane proteins resembled that of A . rnediterrunea, the

source of the nucleus. The converse results were also obtained from a reciprocal

transplantation experiment. This indicates that genes for some chloroplast membrane proteins and some ribosomal proteins in Acetabularia are in the nuclear

genome and others may be located in the plastome.



The genetic backgrounds of many chloroplast mutations with defects in photosynthetic CO, fixation have been described (von Wettstein and Kristiansen,

1973). Most chloroplast mutants investigated so far in higher plants do not



survive when grown in the field; the stocks are therefore maintained in the

heterozygous condition, and in all cases the progeny arising from a heterozygous

plant exhibit a Mendelian segregation of three wild-type to one mutant seedling.

The homozygous mutants are easily identified by color; they are all yellow or

paler green than wild-type plants. Using cytogenetic analysis of pea mutants of

chlorina type, Blixt (1968) identified 15 nuclear genes controlling the formation

of plastid pigments. An extensive genetic analysis of 198 induced recessive lethal

chloroplast mutations in barley (Hordeurn vulgare) revealed that there are 86

nuclear genes responsible for the regulation of chloroplast development (von

Wettstein et al., 1971). The 12 mutants represent mutations in 12 different

nuclear genes. Five of them (xantha-f, g, h, 1, n ) were classified as structural

genes responsible for the conversion of uroporphyrinogen to protochlorophyllide;

three genes (tigrina 12, “infrared” 5 and 6 ) were identified as possessing

regulatory functions. Unlike struatural genes, two regulatory genes exhibited a

similar dominant effect in wild-type and mutant alleles in heterozygotes. The

positive correlation between chlorophyll contents and photosynthetic rates on a

gross chlorophyll basis among five barley mutants (viridis-b, 1, d, c, and rn)

suggests that the reduced absorption of light by the lower amounts of chlorophyll

in these mutants is the major factor responsible for their reduced photosynthetic

rates on a gram fresh weight of leaves (Carlsen, 1977). On the contrary, if a

mutant is characterized by a higher photosynthetic rate in bright light than the

wild type, as has been observed in the case of an exceptional mutant viridis-k

(Carlsen, 1977), such a result could possibly be interpreted on the basis of

intergenomic interaction leading to superior biochemical quality of light-harvesting chlorophyll in the photosynthetically efficient mutant chloroplast.

Recognizable mutant plastids have been found in the subgenus Oenothera

(Stubbe, 1957), and they could be distinctly identified by the pattern of variegation. Oenothera suaveolens had white defective plastids in some leaf areas,

yellow-green defective plastids in other leaf areas, and normal green ones in still

others. In some complex Oenothera hybrids there were plants that had all three

types of the suaveolens plastids along with normal plastids from Oenothera

lamarckiana. Renner ( 1919) first demonstrated that variegation, seedling lethality, or embryo abortion of certain species hybrids were due to incompatibilities

between the plastome and nuclear genomes. On the basis of these incompatibilities and other general cytogenetical observations, Kutzelnigg and Stubbe

(1974) have recognized five chloroplast genomes (plastome I-V) in subgenus

Oenothera, each of which is adapted to distinct nuclear genome class. The close

complementation of the nuclear and plastome genes in Oenothera has aroused

interest among investigators to devise model experiments at the molecular level

to make a distinction between the five plastomes of subgenus Oenothera in

respect of their genetic codes. The hypothesis that intergenomic interaction between different allelic forms of nuclear and plastome genes plays a major role in



the evolution of various Oenothera species is attractive but nevertheless demands

specific experimental evidence.


The use of ribulose- 1,5-biphosphate carboxylase/oxygenase (RBPCase, EC as a marker enzyme of both nuclear and chloroplast genomes in genetical experiments at the subcellular level in higher plants has recently been advocated. This carboxylase comprises up to 50% of the total soluble protein in

leaves, and thus is the most abundant protein in nature. RBPCase in the chloroplast of higher plants participates both in the photosynthetic carbon reduction

cycle and in the photorespiratory carbon oxidation cycle (Jensen and Bahr,

1977). In photosynthesis the enzyme catalyzes the only reaction known to give a

net increase in the amount of carbon compounds by fixing a molecule of CO, to

generate 2 mol of glycerate 3-phosphate, while during photorespiration it

functions as an oxygenase, converting ribulose-l,5-biphosphate into glycerate

3-phosphate and phosphoglycolate. The two molecules of glycolate thus yielded

are converted in the peroxisomes and mitochondria (Lorimer et al., 1977) into

glycerate 3-phosphate with the release of a molecule of CO,. The glycerate

3-phosphate reenters the carbon reduction cycle, making more C 0 2 available for

fixation by the carboxylase reaction. The dual function of RBPCase seems to

protect chloroplasts against temporary COPdeprivation in the light: it permits the

internal generation of C02, the dissipation of harmful photochemical energy

through the production of glycolate by oxygenation, and the partial recovery of

the diverted carbon.

Wildman and co-workers in their pioneer work on the physical chemistry of

leaf proteins first discovered this large molecular fraction 1 protein (MW

550,000). The RBPCase molecule is an oligomer of 16 subunits; 8 are termed

large (MW 55,000) and carry the catalytic site (Nishimura and Akazawa, 1973),

while the other 8 are termed small subunits (MW 12,000-15,000). In addition to

the two major products, about 90 other minor products of the enzyme have been

separated from isolated pea chloroplasts by electrophoresis on one- and twodimensional polyacrylamide gels (Highfield and Ellis, 1978). One major product

is tightly bound to the chloroplast lamellae, and is associated with the ATPsynthase complex. The other major product is soluble, and is identified as the

large subunit of RBPCase, the key enzyme in photosynthesis and photorespiration. Peptide mapping and characterization by isoelectric focusing (Kung, 1976;

Sakano et al., 1974) of the subunits from Nicotiana species and their respective

F, hybrids have shown that the large subunit is maternally inherited and that the

small subunit is inherited in a Mendelian fashion. The large subunit is encoded in

ct-DNA in maize (Coen et al., 1977), and Chlamydomonas (Gelvin et al.,



1977), and is synthesized from a messenger RNA lacking poly(A) (Highfield and

Ellis, 1978). A ct-DNA fragment from maize generated by restriction endonuclease was cloned and directed the synthesis of the large subunit in an in vitro

linked transcription-translation experiment. Messenger RNA for the large subunit has been isolated from chloroplast polyribosomes in Chlarnydornonas with

the aid of antibodies and was shown to hybridize with ct-DNA. By contrast, the

small subunit is encoded in nuclear DNA; this subunit is not labeled when

isolated chloroplasts incorporate labeled amino acids into protein, but it has been

identified as an in vitro translation product of cytoplasmic ribosomes (Roy et al.,

1976). The small subunit is synthesized on cytoplasmic polyribosomes only in a

precursor form (Dobberstein et al., 1977). The precursor form contains an

additional amino acid sequence of 44 amino acids at the N-terminus that is

cleaved off by an endoprotease in connection with its transfer across the chloroplast membrane.

The involvement of intergenomic interaction therefore is envisaged in the

formation of a complete and functional molecule of RBPCase having catalytic

roles for both photosynthesis and photorespiration. The recent data on the primary

structure of RBPCase from a number of plant species confer interpretations in

terms of evolution, adaptation, and function of this key enzyme. The marked

variation in the N-terminal sequence of the 110-120 amino acids of the small

subunit found in different plant species using the automatic Edman degradation

technique reveals the existence of allelic polymorphisms of the nuclear genome

(Gibbons et al., 1975; Poulsen et al., 1976). Nuclear gene polymorphism has

also been reported for the C-terminal end sequences of the small subunit as

determined by carboxypeptidase digestion (Sugiyama and Alcazawa, 1970;

Strobaek et al., 1976; Poulsen, 1977) in spinach (-Phe-Leu-Thr-Tyr-COOH),

tobacco (-Thr-Val-Leu-Tyr-COOH), and barley (-Leu-Tyr-Phe-Val-Asn-AlaCOOH). Sequence information on the large subunit is so far only available for

barley and spinach (Poulsen, 1978; Stringer and Hartman, 1978). The large subunit of barley contains 9 methionine residues and yields up to 10 fragments after

cyanogen bromide cleavage. Six fragments accounting for about 350 of the 490

residues have been successfully separated and partially sequenced. Comparison

of the corresponding sequences from both barley and spinach has revealed 4

amino acid replacements among 35 analyzed residues (Stringer and Hartman,

1978). The results indirectly suggest species differences in the nucleotide sequence of the plastome genes exclusively responsible for coding the large subunit

of RBPCase.

The intergenomic complementation as evidently involved in the biosynthesis

of RBPCase and the occurrence of polymorphic forms of this enzyme in many

plant species have led investigators to uncover the specific nature of “genomeplastome” interaction in the subgenus Oenothera (von Wettstein et a l . , 1978).

An isolation procedure for Oenothera RBPCase and a peptide mapping proce-



dure to characterize the subunit polypeptides have been worked out (Holder,

1976, 1978). In order to demonstrate whether the five classes of chloroplast

genomes of Oenothera subspecies in the classification scheme suggested by

Kutzelnigg and Stubbe (1974) carry different allelic forms (nucleotide sequences) of the genes for the large subunit of RBPCase, von Wettstein et al.

(1978) intensified their efforts to precisely define the peptide sequences of the

isolated S-carboxymethylated large subunit of the enzyme digested with chymotrypsin. The comparative data on the large subunit chymotryptic peptide maps of

nine Oenothera subspecies are summarized in Table IV. Some marked variation

in terms of the presence or absence of four peptides designated A, B, C, and D in

the five plastome groups of Oenothera has been observed. Although the detectable differences in the peptide maps of the large subunit of RBPCase from the five

plastome groups of Oenothera remains small, the results reveal the occurrence of

single amino acid replacements among the plastome groups, reflecting nucleotide

sequence variation and thus evolutionary divergence of Oenothera subspecies

based on plastome genes. The differences in plastome genes among Oenothera

subspecies could have been produced due to nucleotide substitutions and recombination leading to allelic polymorphism of chloroplast genomes. The large

RBPCase subunit proteins are similar or identical for an Aegilops speltoides

accession, the tetraploid wheats, T. dicoccum, T. turgidum, T . timopheevi, and

the hexaploid wheat T. aestivum, but are distinctly different in electrophoretic

mobility from those of T. boeticum, T. monococcum, T . urartu, and Ae. squarrosa (Chen et al . , 1975). Their results (albeit based on single accession of each

Table IV

Peptide Maps of the Large Subunit of Ribulose-l,5-biphosphate

Carboxylase/Oxygenase from Nine Oenotheru Subspecies


Chloroplast genome"

Peptide mapsb

0 . hookeri

0 . strigosu

0 . elata

0 . biennis (Miinchen)

0 . biennis (Citronelle)

0 . lamarckiana

0. parviflora (ammophila)

0 . purvifloru (atrovirens)

0 . urgillicolu

Plastome I

Plastome I

Plastome I

Plastome I1

Plastome 111

Plastome I11

Plastome IV

Plastome IV

Plastome V

A- B- C+ D+

A- B- C- D+

A- B+ C+ D+

A- B+ C+ D+

A- B+ C+ D+

A- B- C- D+

A+ B+ C+ DAt B+ C+ DA+ B- C+ D-

"The classification of chloroplast genomes is according to Kutzelnigg

and Stubbe (1974).

bAdapted from von Wettstein er ul. (1978). The superscript + or - after

each letter indicates the presence or absence of four chymotryptic peptides in the large subunit of the enzyme.



species) suggest that the plastome genes are similar for Ae. speltoides and cultivated polyploid wheats, supporting other evidence that an Aegilops species similar to Ae. speltoides may be the B genome progenitor of modem hexaploid wheat.



Modem advances in plant cell culture demonstrate that protoplasts from leaf

mesophyll cells can be induced to regenerate into entire plants, and that they can

be stimulated to fuse by defined experimental manipulations. A combination of

these two techniques permits the fusion of protoplasts isolated from two different

species and the regeneration of a somatically produced hybrid plant. The two

major objectives of producing parasexual hybrids through protoplast fusion in

higher plants are ( i ) the creation of new hybrid plants having mixed populations

of mitochondria and chloroplasts in their cells, which cannot be achieved sexually because of the unequal or negligible contribution of cytoplasm from the

paternal gametes, and ( i i ) the creation of new plant species by somatic fusion of

protoplasts that cannot be sexually hybridized-a creation that could not have

been visualized until very recently. The first parasexual hybrid derived from the

fusion of protoplasts of Nicotiana glauca and Nicotiana langsdofii was obtained by Carlsen et al. ( 1 972). Analysis of the polypeptide composition of RBPCase from this hybrid shows that the polypeptide of the large subunit is similar to

that of N . glauca (Kung et al., 1975; Kung, 1976). The large subunits of N .

langsdofii in the hybrid were not detected, suggesting that the plastome coding

for the large subunit of RBPCase from N . langsdofii is not expressed. However, the hybrid contains the small subunit of both species, indicating that both

nuclear genomes are equally expressed in the hybrid. The explanation as to why

only one chloroplast genome was transmitted to the hybrid for expression of the

large subunit of RBPCase is not yet fully available. Part of the answer comes

from the work of Smith et al. (1976), who produced 23 mature hybrid plants by

the fusion of protoplasts of N . glauca and N . langsdofii. In a total of 20 hybrid

plants analyzed, 12 of them have the plastome of N . langsdofii expressed,

whereas 7 have only the plastome of N. glauca expressed. There was, however,

one hybrid out of a total of 20 plants in which chloroplast genomes of both

species were equally expressed. When the plastome of either species is expressed, the hybrid exhibits the characteristic tissue morphology similar to that

derived from sexual hybridization. In the case where plastomes of both species

are expressed, the hybrid plants appear abnormal. This is probably caused by

incompatibility of interaction between chloroplast and nuclear genomes. The

result of such incompatible interaction between the combined plastomes and

either one of the two nuclear genomes may probably lead to some disturbance of

normal metabolic activity and hence abnormal plant development. More study



from a large plant population is, however, required to resolve the mystery of

uniparental or biparental transmission of organelle genomes in higher plants.

Somatic hybrid plants of potato (Solanum tuberosum) and tomato (Lycopersicum esculentum) from fused protoplasts have been successfully produced (Melchers et d.,1978), and ingenious biochemical analyses of RBPCase as

phenotypic markers of chloroplast and nuclear genomes from the resulting intergeneric hybrids offer for the first time some clues regarding allelic differences

between plastome genes for the large subunit as well as between nuclear genes

for the small subunit. The variation patterns based on isoelectric focusing of

small subunit bands from four tomato-potato hybrids (6a, 6b, lb, 7a) together

with those of tomato and potato for comparative purposes are presented in Fig. 1.

It can be seen that all four hybrids contain the three prominent tomato small

subunit bands. In addition, they all contain the prominent potato small subunit

band and two of the minor bands. One of the minor potato small subunit bands is

absent in all the hybrids, while the other minor band is present in only two of the

four intergeneric hybrids-6b and 7a. These results reveal that the native RBPCase in the hybrids contains the small subunit products resulting from the expression of both tomato and potato nuclear genes, as could be expected in parasexual

hybrids between potato and tomato following Mendelian inheritance. The relative discrepancy for the presence or absence of the four minor potato small

FIG. 1. Isoelectric focusing of S-carboxymethylated RBPCase small subunit from tomato,

potato, and their hybrids. (T)tomato, (P) potato, (6a, 6b, Ib, and 7a) interspecific hybrids. Redrawn

from Melchers er a / . (1978).



subunit bands as observed in the case of the hybrids, however, remains unknown. The RBPCase large subunit polypeptides from the hybrid plants are also

compared with the tomato and potato large subunit polypeptides in Fig. 2. For

each species, the large subunit exhibits two distinct major bands together with at

least three less identifiable minor bands in the original photograph (Melchers et

al., 1978). In three of the four plants (6a, 6b, and lb), the large subunit polypeptides follow the banding pattern of tomato, indicating uniparental transmission of

chloroplast genes in these somatic hybrids. The position of the densely stained

major bands of the large subunit from the fourth hybrid (7a) is, however, identical to those of the large subunit from potato. These results were interpreted by the

authors to mean that in three out of four hybrids the functional ct-DNA was

transmitted from tomato, whereas in the fourth the ct-DNA was contributed from

potato. Although the genetic interpretation of these findings remains restricted

for lack of more data from a significant number of tomato-potato mature hybrids,

the results of Melchers et al. (1978) nevertheless indicate that the products of

both tomato and potato nuclear genomes are present in the RBPCase oligomer,

whereas the large subunit products are evidently derived from either tomato or

potato, i.e., uniparentally. Why should there be expression of plastome genes

from only one of the parents during crossing or protoplast fusion? If by sexual

hybridization or parasexual protoplast fusion two chloroplast genomes are

brought into one cell, why then does only one or the other organelle genome win

out and functionally establish itself during the development of the individual? It

is not known if a particular type of plastome or ct-DNA produces a repressor

molecule against the other or whether plastome competition and its functional

preference for a specific nuclear genome is responsible for the perpetuation of

large Subunit












FIG. 2. Isoelectric focusing of S-carboxymethylated RBPCase large subunit from tomato,

potato, and their hybrids. (T)tomato, (P) potato, (6a, 6b, Ib, and 7a) interspecific hybrids. Redrawn

from Melchers er al. (1978).



ct-DNA from only one of the parents in the hybrid. It is likely that the four

parasexual hybrid plants so far analyzed for RBPCase were aneuploids rather

than true amphidiploids. The other cause of the predominant representation of

tomato ct-DNA in three of the hybrids may have been due to the differentiated

state of the tomato chloroplast at the time of fusion; the potato protoplasts, on the

contrary, contained undifferentiated protoplastids during protoplast fusion (Melchers et d . , 1978). Chiang (1968) has reported the results of experiments in

Chlamydomonas using parents pregrown with 3H/'4C-labeled adenine to distinguish the parental ct-DNAs in zygotes, and later (1971) using both CsCl density

and radioisotope labeling simultaneously. He reported equal contribution of label

from both parents in DNAs extracted several days after mating and from zoospores after meiosis and zygote germination, and concluded that ct-DNAs from

both parents were conserved in the zygote. Human-rodent hybrid somatic cell

lines were examined for the presence of human and rodent mt-DNA, separable

by buoyant density in gradients, and identified by hybridization to complementary RNA (Horak et al., 1974). Of the lines examined, 13 out of 18 showed the

presence of both human and rodent mt-DNA sequences covalently linked and

hence probably combined by recombination. In other studies by Wallace et al.

(1976) hybrids were formed between human chloramphenicol-resistant (mitochondrial capr) and mouse chloramphenicol-sensitive (caps) cell lines. Some

of the hybrid lines exhibited growth in the presence of chlorarnphenicol

and were classified as cap' like the human parent cells, but contained only or

predominantly mouse mt-DNA sequences as judged by hybridization to complementary RNA. Such cell lines could have been produced by recombination,

but they might also have resulted from selection of capr mutations in the mouse

mt-DNA molecules.




There seem to be no set rules at the cellular level for organelle DNA transmission. In oogamous species (gametes from male and female parents differ

morphologically) showing purely maternal inheritance, the paternal organelles

may be excluded at any step in the reproductive processes (Paolillo, 1974;

Hageman, 1976; Birky et al., 1978). Paternal organelles are visibly destroyed in

the zygotes of some mammals and algae (Bandlow er al., 1977; Kirk and

Tilney-Bassett, 1978). Plants showing biparental inheritance of chloroplast genes

almost always have a strong maternal bias; the single exception is the Japanese

sugi, Cryptomeria japonica. This tree, the only gymnosperm studied in much

detail, exhibits a great preponderance of paternal zygote, a few maternal zygote

and biparental zygote, and an overall bias in favor of paternal chloroplasts (Ohba

et al., 1971). In the four thoroughly studied species that show biparental inher-



itance of organelle genes, Chlamydomonas, yeast, Pelargonium, and Oenothera, there is good reason to believe that most or all zygotes receive intact

organelles from both parents, and degeneration of entire organelles has not been

seen in the zygotes of Chlamydomonas (Cavalier-Smith, 1970), Oenothera

(Meyer and Stubbe, 1974), and yeast (Osumi et a l . , 1974).

Two pertinent questions could be asked in respect of the precise molecular

mechanism of organelle DNA transmission: ( a ) What happens to the mitochondrial or chloroplast genomes that do not appear in the progeny of uniparental

transmission? ( b ) Why does the same crossing or mating combination produce

both maternal and biparental transmission, and sometimes paternal transmission

as well? The validity of the biparental transmission of ct-DNA as demonstrated

by Chiang (1968, 1971) has, however, been questioned by Sager (1977), who is

of the view that some nuclear factor produced by the maternal parent is required

for effective degradation of the paternal cytoplasmic genome, i.e., for maternal

inheritance. Sager’s model is based on an analogy with bacterial host modification restriction systems. Ct-DNA from the mt+ (“maternal”) and mt- (“paternal”) parents is supposed to be differently marked, possibly by methylation of

the mt+ ct-DNA, prior to fusion of the parental chloroplast in the zygote. A gene

closely linked to the mt+ allele produces a restriction enzyme that specifically

destroys the unmodified mt- ct-DNA. Destruction or degradation is complete in

most zygotes, which consequently transmit only chloroplast genes from the mt+

parent to their progeny and are therefore referred to as “maternal” zygotes even

though the Chlamydomonas is an isogamous species (gametes from both parents

are alike morphologically). Degradation of mt- ct-DNA fails to occur or is

incomplete in rare zygotes, which can then transmit both mt+ and mt- chloroplast

genes and are biparental zygotes. Paternal zygotes, which transmit only mtchloroplast genes, are found in significant numbers after treatment of the mt+

gametes with inhibitors or in the presence of the mat-I gene mutation. In an

attempt to obtain physical evidence for degradation of the ct-DNA from the mtparent in zygotes in support of the model, Sager and Lane (1972) labeled parental

ct-DNAs differentially with [lsNP4N]- or [3H/’4C]adenine, and were unable to

detect the paternal ct-DNA on zygotes; they also found a density shift in the

maternal ct-DNA, such as might result from methylation. Schlanger and Sager

(1974) report that when the mt+ gametes are irradiated with UV the mt- parental

ct-DNA is conserved and mt+ ct-DNA is partially or completely degraded, corresponding to the enhanced transmission of mt- markers and reduced frequency of

maternal zygotes. Chiang (1976) has verified his own results (1968; 1971) by a

set of newly designed experiments in Chlamydomonas, and the recent results

suggest that up to 95% of both parental ct-DNAs is degraded in zygotes, but the

paternal ct-DNA is degraded more rapidly. This degradation occurred simultaneously with replication and recombination of mt+ and mt- ct-DNAs. Birky

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IV. Genetic Implications of Intergenomic Interactions

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