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� 2000 The American Genetic Association 91:435–440

Erwin Baur or Carl Correns: Who ReallyCreated the Theory of Plastid Inheritance?Rudolf Hagemann

Historical reviews of the field of non-Mendelian genetics and many other publica-tions credit Erwin Baur and Carl Correns equally for the development of the theoryof plastid inheritance. However, a study of the original literature indicates that thisconclusion is not correct. Analysis of the relevant articles leads to the conclusionthat Baur alone deserves credit for the theory of plastid inheritance. In his classicarticle on the inheritance properties of white-margined Pelargonium plants, Baur(1909) stated: (1) The plastids are carriers of hereditary factors which are able tomutate. (2) In variegated plants, random sorting-out of plastids is taking place. (3)The genetic results indicate a biparental inheritance of plastids by egg cells andsperm cells in Pelargonium. By contrast, Correns held the view that in variegatedplants there is a maternally transmitted labile state of the cytoplasm which switcheseither to a permanently ‘‘healthy’’ state (allowing the ‘‘indifferent’’ plastids to be-come green chloroplasts) or to a permanently ‘‘diseased, ill’’ cytoplasmic state(causing white plastids and cells). Otto Renner supported Baur’s theory and workedout important characteristics of plastid inheritance in the genus Oenothera. In the1930s Renner reported many more observations, which established plastid inher-itance as a widely accepted genetic theory.

Plastid Genetics as an IntegralPart of Current Genetic Research

Research involving plastid as well as mi-tochondrial genetics has contributed torecent advances in both basic and appliedresearch. Over the past 15 years the com-plete DNA sequences of several plastid ge-nomes from higher and lower plants havebeen determined. In addition, extensivestudies on the expression of plastid genes,their mutations, their regulation, and in-teraction with the nucleocytoplasmiccompartment have advanced our knowl-edge of the role of plastid genes within thegenetic system of plants.

Recent molecular work also has provid-ed new insights into the evolution of plas-tids from formerly free-living cyanobacte-ria. Nowadays a wealth of genetic,biochemical, and cytological data supportthe idea that the endosymbiotic uptake ofa cyanobacterium by a pre-eukaryotic cellwas followed by the gradual transforma-tion of the endosymbiont into a plastidlikecell organelle—a process accompanied bymassive gene transfer from the plastid ge-nome to the chromosomes in the cell nu-cleus. In addition to their importance in

basic research, plastids have also attract-ed the attention of plant biotechnologistsdue to the recent development of trans-formation technologies for higher plantchloroplasts, which enable the expressionof foreign genes in the plastid compart-ment (Bock and Hagemann 2000).

In contrast to the general importance ofresearch on plastid genetics, many text-books of genetics (irrespective of theircountry of origin) almost completely ig-nore the field of plastid genetics: it is men-tioned on at most two or three pages in atotal text of 500 or 800 pages and, more-over, the text frequently contains errone-ous statements. (The situation regardingmitochondrial genetics is slightly—butnot much—better.)

This article addresses this problem byproviding a review of the original litera-ture from the turn of the 20th century.This summary of the German literaturemay also be useful to scientists within thediscipline of organelle genetics who havequestions concerning the history of thisfield, how this discipline has evolved fromits very beginning, and who was the dis-coverer of the basic principles of plastidinheritance.

From the Institute of Genetics, Martin-Luther-Universi-ty, Weinbergweg 10, D-06099 Halle/Saale, Germany. Iwish to thank Dr. Ralph Bock for valuable discussionsof the manuscript and his advice for improvement ofthe text. Moreover, I thank Dr. Richard Tilney-Bassettand three anonymous referees for critical reading ofthe manuscript and many valuable comments.

436 The Journal of Heredity 2000:91(6)

Figure 1. Erwin Baur (1875–1933).

Discovery of Non-Mendelian(Extranuclear) Inheritance by CarlCorrens and Erwin Baur

In one issue (no. 3) of Volume 1 of the‘‘Zeitschrift fur induktive Abstammungs-und Vererbungslehre’’ (the first interna-tional journal of genetics and the progen-itor of Molecular and General Genetics),Correns and Baur separately published ar-ticles on non-Mendelian inheritance of var-iegated phenotypes in plants. Correns(1909a) emphasized in a footnote to hisarticle: ‘‘The simultaneous publication ofthis paper with the following article by E.Baur is based on mutual agreement; how-ever, each of us had no knowledge of thecontents of the other ones paper.’’ The ti-tles of the two articles are

Correns, Carl. Vererbungsversuche mitblass(gelb)grunen und buntblattrigen Sip-pen bei Mirabilis jalapa, Urtica piluliferaund Lunaria annua. Zeitschr f ind Abst uVererbungsl 1909;1:291–329. ( Inheritanceexperiments with pale (yellow) green andvariegated varieties of Mirabilis jalapa, Ur-tica pilulifera and Lunaria annua.)

Baur, Erwin. Das Wesen und die Erbli-chkeitsverhaltnisse der ‘‘Varietates albom-arginatae hort.’’ von Pelargonium zonale.Zeitschr f ind Abst u Vererbungsl 1909;1:330–351. (The nature and the inheritanceproperties of horticultural varieties of Pel-argonium zonale having white borders.)

For different plant species both authorsreported the non-Mendelian mode of in-heritance of green-white or green-yellowleaf variegations.

Following crosses between variegated,yellow, and green plants (or branches) ofMirabilis, Urtica, and Lunaria, Corrensfound that the leaf color trait (green ver-sus yellow) is inherited through the moth-er only, that is, that there is a purely ma-ternal inheritance: green branches alwaysproduced seeds that gave rise to greenseedlings, seeds from yellow branchesyielded only yellow offspring, while thevariegated branches produced green, yel-low, and green-yellow variegated seedlingsin widely varying ratios. In contrast, thepollen-providing parent had no influenceon the phenotype of the progeny.

Baur found another type of non-Mende-lian inheritance in his crossing experi-ments with P. zonale. He crossed greenand periclinal chimeric white-marginedplants (or white shoots from otherwisewhite-margined plants). Reciprocal cross-es revealed a biparental, but non-Mende-lian, inheritance of the leaf color trait

green versus white. The F1 progeny con-sisted of green, green-white variegated,and white seedlings. In many crossesthere was a bias in the F1 toward the phe-notype of the maternal parent.

Thus, in 1909, Baur and Correns simul-taneously discovered and described theoccurrence of non-Mendelian inheritancein higher plants. Their conclusion that inaddition to the Mendelian inheritance ofgenes in the cell nucleus, there are otherhereditary factors outside the nucleus(i.e., in the protoplasm) which exhibit anon-Mendelian mode of inheritance marksa milestone in genetics and the date ofbirth for a new field of research. Althoughcredit for the initial observations shouldbe shared between Baur and Correns,their subsequent theoretical interpreta-tions distinguish their contributions to thefield.

The Foundation of the Theory ofPlastid Inheritance

The authors of numerous monographs, re-view articles, and textbooks express theopinion that, in 1909, Erwin Baur and CarlCorrens contributed equally to the estab-lishment of the theory of plastid inheritance.However, a careful study of the original lit-erature indicates that this interpretationdoes not reflect the history of plastid genet-ics correctly. Below, the original sources inthe genetic literature are analyzed.

The Position of Erwin BaurThe theory of plastid inheritance and of ran-dom sorting out of plastids. In his articleBaur (1909) was the first to provide theexplanation that chloroplast defects in P.zonale and their mode of inheritance arecases of plastid inheritance: The greenand the white shoots of variegated plantsdiffer in the genetic constitution of theirplastids (Baur used the term ‘‘chromato-phores’’). One plastid type is green (nor-mal, nonmutated, capable of becominggreen during ontogenetic development);the other type is white (mutated, incapa-ble of becoming green).

In his 1909 paper, Erwin Baur (Figure 1)explained the green-white variegation ofseedlings and the formation of branches inthe F1 plants with different phenotypes(green, variegated, white; periclinal or sec-torial chimeras) by the theory that somat-ic segregation and sorting out of plastidshad occurred: variegated plants devel-oped from zygotes containing two distincttypes of plastids, namely green plastids(from one parent) and white plastids

(from the other parent). In the course ofthe subsequent cell divisions, plastids seg-regate randomly. This random segregationregularly results in sorting out of the twotypes of plastids and—during ontogeneticgrowth of the plants—in the formation ofpure green and pure white areas in oneand the same plant.

Baur’s development of this theory isdocumented by the following citationsfrom his 1909 paper (in English transla-tion):

I have, on the basis of these observations,formed a theory which shall serve as a workinghypothesis for the following investigations [. . .]The P. zonale races with white-margined leavesare chimeras with a periclinally divided growingpoint.. The peripheral two or three cell layersof the shoot apex have only albino plastids,which entirely lack the capacity of chlorophyllformation. The central cells of the shoot apex,however, contain entirely normal plastids ca-pable of turning green.

The fertilized egg cell, as generated throughthe union of a ‘green’ with a ‘white’ gamete cell,thus contains two types of plastids, green andwhite. During the cell divisions leading from theegg cell to the embryo, the plastids are distrib-uted to the daughter cells entirely randomly.Whenever a daughter cell receives only whiteplastids, this cell subsequently will have onlywhite descendent cells, and will give rise to awhite sector within the mosaic; whenever adaughter cell receives only green plastids, a sol-id green sector will arise from it. Cells with bothtypes of plastids will be capable of segregatingfurther, and so on.; probably, I need not explainthis in greater detail. If the cell, which will giverise to cotyledons and the growing point, re-ceives only green plastids during one of the firstdivisions, then a seemingly pure green hybridwill be produced. If only white plastids segre-gate into that cell, seemingly white progeny willbe generated, etc. Furthermore, a simple con-sideration—which I need not explain through

Hagemann • Baur or Correns: Who Really Created Plastid Genetics? 437

Figure 2. Carl Correns (1864–1933).

lengthy or detailed writing—leads us to realizethat, after a large number of divisions, cells withboth types of plastids will decrease in percent-age as compared to cells with only one type ofplastid.

This hypothesis is well in accordance with allfacts that, thus far, have been determined em-pirically; it will have to be tested by further in-vestigations which, however, I do not at all wishto reserve for myself.

[Original German text in Baur (1909:349–351)and in Hagemann (2000:102). Baur always usedthe term ‘‘chromatophores,’’ which has beentranslated into ‘‘plastids.’’]

Hypothesis of biparental transmission ofplastids in Pelargonium. In the first decadeof the 20th century, among botanists, theopinion was held that in higher plants theplastids are transmitted to the next gen-eration only by the mother plant. Baur im-mediately realized the inconsistency of hisexperimental genetic findings with thiswidely held opinion. Trusting his experi-mental results he proposed that his cross-es might indicate that, in Pelargonium,plastids are transmitted biparentally tothe next generation (i.e., by the female aswell as by the male parent):

The hypothesis makes—to emphasize this onceagain—just one assumption, that is not yetproven, namely that the fertilized egg cells gen-erated through fusion of a ‘white’ with a ‘green’gamete cell, carry two types of plastids: whiteones and green ones. [. . .] According to the cur-rently dominant scientific dogma, the plastidsof the fertilized egg cell descend only from themother; whether or not this is really absolutelycertain, shall remain undiscussed here. If theplastids should indeed always come only fromthe mother and, hence, if the currently domi-nant opinion is correct, then we are faced herewith extremely curious modes of inheritance. Itthen must be the case that, in a white � greencross, part of the white plastids of the egg cellare converted into green plastids under the in-fluence of the male sexual nucleus, and, in thereciprocal cross, part of the green plastids ofthe egg cell must turn into white plastids underthe influence of the male sexual nuclei stem-ming from a white plant. Of course, theoretical-ly, something like that can be imagined, how-ever, to the best of my knowledge, there is noinheritance phenomenon known which wouldbe analogous to this in any respect.

But should it—in contrast to the hitherto rul-ing doctrine—turn out that the male spermcells are also able to transmit plastids into theegg cells, then the hereditary processes of thewhite-margined plants (of Pelargonium) wouldbe fully understandable.

Only further investigations will enable dis-crimination among these possibilities. It is def-initely necessary that the developmental pro-cesses regarding the plastids of higher plantshave to be carefully studied with newer meth-ods in a continuous series from the sexual cell(through the developing organism) to the sex-ual cell of the next generation.

[Original German text in Baur (1909:350) and inHagemann (2000:103).]

In his following publications, Baur(1911, 1919) described several additionalexperiments and observations from hisstudies with Pelargonium zonale whichwere in full agreement with his initial sug-gestions outlined in the article of 1909.

Studies on green-white variegated plantsof Antirrhinum majus. Simultaneously withthe Pelargonium studies, Baur performedexperiments with green-white variegatedplants of snapdragon (Antirrhinum majus)(Baur 1910a,b). In this species he found(by conducting reciprocal crosses) thesame mode of non-Mendelian inheritanceas Correns (1909a,b) had described forMirabilis jalapa: In A. majus, the differencebetween green and white is inheritedthrough the mother only: green branchesgive rise to green seedlings only, whitebranches produce only white offspring,while the variegated branches producegreen, white, and variegated seedlings inwidely varying ratios. The genetic consti-tution of the pollen had no effect on thephenotype of the progeny.

Initially Baur only pointed out the ac-cordance of his findings with the resultsthat Correns had obtained for Mirabilis.However, in the third edition of his text-book of genetics, Baur (1919) clearly stat-ed that the examples of green-white var-iegation in Pelargonium (biparentalinheritance) as well as in Antirrhinum, Mir-abilis, and Primula (maternal inheritance)are cases of plastid inheritance. Baur re-ferred to, accepted, and supported theview of O. Winge (1919) that variegation inthe aforementioned species are examplesof plastid inheritance. The only differenceis that in the case of maternal inheritance,the plastids are transmitted only by theegg cells, whereas, in the case of biparen-tal inheritance (Pelargonium), the plastidsare transmitted by both the egg cells andthe sperm cells of the pollen. Baur heldthis view in all of his later publications onthis issue.

Although Thomas Hunt Morgan wasvery skeptical regarding many reports onthe phenomenon of ‘‘cytoplasmic inheri-tance,’’ in his book The Physical Basis ofHeredity, Morgan (1919) expressed hisagreement with the theory that plastidsare carriers of hereditary factors.

The Position of Carl CorrensBy contrast with the interpretations ofBaur, Carl Correns (Figure 2) developed arather different hypothesis. His line of ar-

gument was based on the terms ‘‘healthy’’and ‘‘ill’’ (‘‘diseased’’). In his articles of1909 (a,b), he expressed the view that thefactor determining green or yellow (orwhite) plastids was due to the relativehealth of the cytoplasm: ‘‘The basis of thewhite leaf color is a diseased state of thecytoplasm, a non-infectious chlorosis.’’Correns assumed the existence of twotypes of cytoplasms, a healthy cytoplasmand an ill one. When (indifferent) plastidsare introduced into a healthy cytoplasm,they develop into normally green chloro-plasts; however, when they are introducedinto an ill cytoplasm, then they remain (orbecome) white or yellow. ‘‘Thus, the cellnuclei of the whole plant would be uni-form and healthy. However, the cyto-plasms, depending on the mosaic, ill orhealthy. [Original German text in Correns(1909a:322).]

Correns was well aware of Baur’s con-trasting argument (1) that the plastidsthemselves are the carriers of the geneticdifferences between ‘‘green’’ and ‘‘white’’and (2) that, in Pelargonium, pollen andegg cells transmit plastids to the next gen-eration, while in Antirrhinum and Mirabilis,only the egg cells transmit plastids. How-ever, he was reluctant to accept Baur’sview. ‘‘I cannot believe that the two exper-imental organisms (Pelargonium—Mirabi-lis) should behave so fundamentally differ-ently in this respect’’ (Correns 1909b:340).

In some publications, Correns (1909b,1928) even compared the ‘‘illness of thecytoplasm’’ in such variegated plants withthe action of the agents causing tubercu-

438 The Journal of Heredity 2000:91(6)

Figure 3. Otto Renner (1883–1960).

losis or with spirochetes. In his later arti-cles on non-Mendelian inheritance, Cor-rens (1922, 1928) added to his hypothesisone important new idea: He assumed thatin plants that later will become green-white variegated, the cytoplasm of the‘‘embryonic cells’’ (i.e., the zygote andearly meristematic cells) is in a ‘‘labile cy-toplasmic state.’’ During early develop-ment of the seedlings, this ‘‘labile state’’switches either to a normal, permanently‘‘healthy state’’ (allowing the formation ofgreen chloroplasts) or to a permanently‘‘diseased state’’ (causing white plastidsand cells).

Correns summarized his view in his lec-ture for the Fifth International Congress ofGenetics in Berlin (September 1927). Heheld this opinion until his death in 1933.

Instead of attributing the primary cause forthe different colors to the chloroplasts and as-suming that the healthy green and the ill whiteplastids are located in a cytoplasm which wouldbe neutral in this respect, one could search forthe cause in the cytoplasm itself that thenwould occur in two different states, a healthyone in which the chloroplasts can turn green,and an ill one which prevents them from green-ing. This assumption would let us understandthe absence of ‘mixed’ cells, particularly in ma-ture tissue, because the behavior of all plastidsalways would be determined by the cell cyto-plasm. Besides this, single exceptions that oc-cur more or less frequently, depending on thespecific cytoplasm, probably can be explained.

However, this theory would have to refrainfrom explaining the mosaic of the final differ-entiated stage by tracing it back to a mosaic ina single cell. Even though we could assume thepresence of the two types of cytoplasm in theinitial cell, an unequal distribution of them ac-cording to that of the two types of chloroplastsis too improbable, a mixing would be expectedto occur all too soon.

By contrast, the cytoplasm of the embryoniccells (in such albomaculata-colored plants)could be in a labile state, capable of adoptingthe healthy or the ill state as a consequence ofreasons that we call ‘random’ (1922). The ge-nome would remain unaltered; however, a cellthat once would be differentiated in the one orthe other way, would retain its characteristiccytoplasm [. . .] It can and will undergo furtherdivisions: depending on whether the decisionabout it is made sooner or later, here or there,closer to or farther away from the definitivestate of the organ, it will give rise to a larger ora smaller area of white or green tissue, from en-tire branches down to single cells.

The purely maternal transmission would oc-cur through the cytoplasm (which is subject toconstant changes) and would give rise to en-tirely green and completely white seedlings.Plastids that are transmitted with the sperm nu-cleus would adjust to the egg cytoplasm anddepending on that would become either healthyor ill. [. . .] The colored seedlings, however,would stem from egg cells that were still in thelabile state (which would be in accordance withtheir position in mosaics of Taraxacum and Se-

necio); already after one of the first divisions ofthe embryo a decision could be made leadingto a clear mosaic.

[Original German text in Correns (1928:144–145)and in Hagemann (2000:111, 113).]

One of the main arguments of Corrensagainst Baur’s theory was the presumedlack of ‘‘mixed cells’’ within variegatedleaves. If somatic segregation of green andwhite plastids would take place at ran-dom, one has to expect (according toBaur’s theory) not only cells with greenplastids and those with white plastids, butalso cells that contain both types of plas-tids. Unless the two plastid types are mu-tually exclusive, ‘‘mixed cells’’ (Mischzel-len) should be found containing bothgreen chloroplasts and white plastids sideby side within one and the same cell.

Correns repeatedly stated that such‘‘mixed cells’’ had not been found in a suf-ficient quantity or not at all. However, Cor-rens cited three authors who had reportedthe finding of ‘‘mixed cells’’; among themwas his coworker Funaoka (1924) who ob-served ‘‘mixed cells’’ in variegated leavesof Stellaria media ‘‘relatively frequently’’(Correns and F. von Wettstein 1937:17, 22).Later, such ‘‘mixed cells’’ were found bylight microscopy and by electron micros-copy in many species. I have summarizedthese findings, including all relevant ref-erences, in several previous reviews (e.g.,Hagemann 1964, 1965).

The Foundation of the Theory ofPlastid Inheritance

In view of these original statements byBaur and Correns in their publicationsfrom 1909 to 1933, we must come to aclear conclusion concerning the founda-tion of the theory of plastid inheritance:The only founder of the theory of plastidinheritance was Erwin Baur (cf. Hagemann2000). With his classic article on P. zonalein 1909 he laid the groundwork for the cur-rently flourishing discipline of ‘‘plastid ge-netics.’’

The Further Development of theTheory of Plastid Inheritance byOtto Renner

Baur’s experiments with Pelargonium werebased on so-called loss mutations (‘‘De-fekt-Mutationen’’), plastid mutations caus-ing an inability of the plastids to becomegreen (in any nuclear background thathad been tested); some of these plastidmutations led to yellow plastids and

leaves, whereas most of them causedwhite plastids and leaves).

Starting in 1922, Otto Renner (Figure 3)published numerous articles on plastid in-heritance. He performed crosses betweendifferent species of the genus Oenothera(subgenus Euoenothera). Their analysisled him to the discovery of a new phenom-enon in plastid genetics: hybrid (plastid)deficiency (‘‘Bastardbleichheit’’). Differentspecies of the genus Oenothera do notonly differ in their genotypes, but also inthe genetic constitution of their plastids(the plastome). Evolution involved notonly mutations in the genome complexes,but also a genetic diversification of theplastomes. The differences in the plasto-mes are manifested by specific plastidtypes being unable to green when inter-acting with certain genome complexes;however, when interacting with their na-tive genome complexes, these plastids de-velop into normal green chloroplasts.

Here, occurrence of a chlorophyll defi-ciency is clearly not due to a plastid ‘‘lossmutation.’’ Instead, during evolution of thegenus Oenothera, ‘‘differentiation muta-tions’’ (‘‘Differenzierungs-Mutationen’’) oc-curred that led—paralleled by nuclear mu-tations during species evolution—togenetic differences in the interactions be-tween the nucleus and the plastids. Thegenetic details of these interactions be-tween different nuclear genomes and fivedifferent types of wild-type plastids sub-sequently were defined by Renner’s for-

Hagemann • Baur or Correns: Who Really Created Plastid Genetics? 439

mer student and coworker W. Stubbe(1960). Moreover, Renner (1936) andSchwemmle (1940) also found ‘‘plastidloss mutations’’ in Oenothera whichcaused white plastids (comparable to thewhite plastid mutants studied by Baur inPelargonium and Antirrhinum).

All results of crosses in the genus Oen-othera convincingly proved the biparentalinheritance of plastids. In Oenothera (andalso in Hypericum), reciprocal crosses be-tween green and white plants or brancheslead to distinct reciprocal differences witha strong bias toward the plastids of thematernal parent. Crosses of green � whiteyield variegated, many green, and (almost)no white seedlings; whereas crosses ofwhite � green yield variegated, manywhite, and (almost) no green seedlings.These results were confirmed with bothwhite plastome mutants and hybrid defi-cient plastids.

In several papers and reviews from 1922to 1937, Renner dealt with different as-pects of plastid inheritance. He always em-phasized that his results are in full accor-dance with the theory of plastidinheritance as formulated by Erwin Baur.Renner (1934) expressed his view in a con-vincingly lucid and expressive style:

The explanation provided by E. Baur (1909) forthe inheritance pattern of the green-white-var-iegated Pelargonium plants is very straightfor-ward and persuasive: The non-nuclear-condi-tioned reactions that lead to chromatophores ofdifferent colors are attributed right from the be-ginning to the plastids themselves. A variega-tion ( � a variegated plant) arises by the pollentube introducing plastids into the egg upon fer-tilization; those plastids differ significantly intheir properties from the plastids of the eggand, during the course of development, the twotypes of plastids are distributed among differentcells and, consequently, among different tissueareas. In the green-white variegated Pelargoni-um plants, under the conditions of the experi-ments, only one type of plastid is healthy andcapable of greening, the other one is always illand incapable of forming chlorophyll.

That is to say: In Baur’s hypothesis, all detailsof the variegation phenomena of the Pelargoni-um type are brought to such a perfect agree-ment with the developmental history of theshoot that one could deduce the cell divisionprocesses at the shoot apex—if they were notknown—from the variegation patterns. In turn,if only the cell division processes were knownand the chromatophores were submicroscopi-cally small, one could infer from the phenomenaof colored-leaf plants, the existence of distinctcarriers of the chlorophylls.

How happy would we be if, in other cases, wewould see only half as clearly the relationshipbetween the hereditary material of the germcell and the phenotype of the mature organism!And this clarity would be clouded by the intro-duction of a labile state of cytoplasm just be-

cause we do not see the plastids creeping outof the pollen tube into the egg cell. That we can-not see this transition means, as compared tothe significance of the variegation phenomena,almost as little as the invisibility of the chro-mosomes in mature germ cells as compared tothe significance of the chromosome numberduring the division of the zygote nucleus.

Skepticism is good, but should be measured!The development of the chromosome theory

of Mendelian inheritance has shown that sci-entists were on the right track that, after havingreached a certain body of evidence, did not fur-ther question every aspect of the importance ofthe chromosomes, but believed in the theory.From their experimental data, these believershave erected the foundation of the current the-ory of inheritance, to which the doubters havecontributed nothing significant ever since.

In my opinion, this evidence for the autono-my of plastids is nowadays so well supportedthat it is worth formulating a theory on this au-tonomy and taking this theory as the basis forfurther deductive treatments of this problem.

From the experiences with the sudden ap-pearance of colorless tissue areas in lines thathad been purely green for generations, one hasto conclude that the plastids can change theirconstitution by themselves, without an influ-ence of the nucleus. This change is practicallyirreversible and I do not know any reason whyone should not call this process plastid muta-tion.

[Original German text in Renner (1934:243, 251)and in Hagemann (2000:124–125).]

In the years 1934–1937, the concept ofplastid inheritance as founded and es-poused by Baur and Renner became a ful-ly established and generally accepted ge-netic theory. This was demonstratedimpressively by the statements of Fritzvon Wettstein (1937), who was a formercoworker and admirer of Correns and be-came his successor as the director of theKaiser-Wilhelm-Institute of Biology in Ber-lin. F. von Wettstein took over the respon-sibility for the posthumous publication ofthe book Nicht mendelnde Vererbung (Non-Mendelian Inheritance) which Correnshad intended to publish as a volume of theHandbuch der Vererbungswissenschaft(Handbook of Hereditary Science). He ed-ited the manuscript very carefully and al-ways tried to do this in accordance withthe initial intentions of Correns. However,to the chapter on non-Mendelian variega-tion, he added the following postscript:

The publications of Renner (1922, 1924, 1929,and above all 1936) have informed us of a hugeamount of experimental material and results onthis subject; the details are dealt with there.

These results make it clear that the plastidspossess characteristic and specific traits and aspecific make-up, which guarantee their geneticindependence of the nucleus. Renner hascoined the term ‘‘plastome’’ for this indepen-

dent element. Furthermore Renner (1936) triesto show that the other cases of non-Mendelianvariegation are consistent with this explanation.The differences result primarily from the modeof transmission of the plastids by the pollentube. It seems that a broad range of plastidtransmission is possible from cases with ampletransmission of plastids by the pollen tube tothose in which no paternal plastids are donatedto the egg cell. The sorting-out occurs with dif-ferent speeds, which may be due to differentproperties of the cytoplasm. [. . .] According toall results that have been published thus far, I(� F. von Wettstein) have come to the conclu-sion that the explanations of Renner are wellfounded for many cases; many reservations,which Correns expounded, can be dispellednow.

[Original German text in Correns and von Wett-stein (1937:42, 45) and in Hagemann (2000:123–124).]

This statement indicates that Fritz vonWettstein had decided to revise Correns’position and to accept the convincing re-sults and the clear arguments of Rennerthus agreeing on Baur’s theory of plastidinheritance.

The Way to Molecular Genetics ofPlastids

The foundation and refinement of the the-ory of plastid inheritance was based onclear-cut genetic results, that is, large-scale reciprocal crosses in several spe-cies, their careful analysis as well as in-tense cytologic investigations thatultimately provided important supportiveevidence. This classical work has beensummarized in detail in a number of com-prehensive reviews (Hagemann 1964,1965; Kirk and Tilney-Bassett 1967, 1978).

At the end of the 1950s a number of re-searchers reported the identification ofspecific DNA in plastids. Since 1963–1964,it generally has been accepted that plas-tids contain their own organelle-specificgenetic information (plastid DNA) whichprovides the material basis for the hered-itary factors of the plastids, termed plas-tome (Kirk 1986).

In the following decades, the plastidDNAs of many plant species were intense-ly characterized regarding their buoyantdensity, GC content, contour length, phys-ical size (kbp) and renaturation behavior.In the 1970s, the discovery of restrictionenzymes and the growing methodologicalspectrum of gene technologists paved theway for the rise of plastid molecular ge-netics. Over the past two decades ourknowledge concerning all aspects of plas-tid biology has benefited immensely fromthe power of molecular genetics.

440 The Journal of Heredity 2000:91(6)

Erwin Baur investigated—as mentioned earli-er—green-white variegations of Antirrhinum ma-jus which exhibited a purely maternal inheri-tance. Our plastid research group at theUniversity of Halle studied several of such var-iegated lines. We obtained one line from Profes-sor Hans Stubbe, Gatersleben, who was a PhDstudent and coworker of Erwin Baur. The mu-tant plastids of this line, designated en:alba-1,carry a specific deficiency in photosystem I,and thus have a total block in photosynthesis.The combination of PCR and SSCP (single-strand conformation polymorphism) analysis,followed by DNA sequencing, revealed that thisplastome mutant has—as compared with thewild-type—a single base-pair substitution at co-don 136 (TAT→TAG) within the plastid genepsaB. This point mutation in plastid DNA pro-duces a new stop codon that leads to a trun-cated PsaB protein, which prevents the assem-bly of a functional photosystem I complex(Schaffner, Laasch, and Hagemann 1995).

Comparable investigations were performedwith the yellow-margined variety ‘‘Mrs. Pollock’’of Pelargonium zonale which had been used al-ready by Erwin Baur’s PhD student Ludwig Roth(1927). This yellow plastome mutant also exhib-its a deficiency in photosystem I (Hagemann1979; Herrmann et al. 1976).

These examples show that the plastome mu-tants, initially investigated by Baur and his co-workers, have become—after many decades—valuable tools for studies in the field of plastidmolecular genetics.

Molecular research on plastid biologycontinues to produce exciting findings andto generate stimulating ideas (Hagemann etal. 1981–1998). As this research proceeds, itshould be borne in mind that this whole de-velopment is founded on the brilliant clas-sical genetic analyzes and the visionaryideas of Erwin Baur and Otto Renner.

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Received February 21, 2000Accepted August 30, 2000

Corresponding Editor: David B. Wagner