D. de Nettancourt

Genetical and ultrastructural aspects of self and cross incompatibility in interspecific hybrids between self-compatible Lycopersicum esculentum and self-incompatible L. peruvianum

SummaryCytological and genetical analyses were made of the breeding system of embryo-cultured interspecific tomato hybrids between L. esculentum and L. peruvianum. It was found that fluorescence techniques and electron microscopy allowed a distinction to be made between pollen tubes inhibited by a unilateral incompatibility reaction and pollen tubes inhibited by a self-incompatibility reaction, after self-pollination of the hybrids or after reciprocal crossing between the hybrid and the parental species. The observed differences, if real and reliable, demonstrate that unilateral incompatibility in esculentum pollen tubes is governed by a single gametophytic factor which is either linked or allelic to the S-locus. This finding is discussed with reference to recent reports that unilateral incompatibility is controlled, in peruvianum styles, by a number of different dominant genes and it is concluded that these dominant genes, the S-locus of self-incompatibility and the gametophytic factor regulating the unilateral reaction in esculentum pollen belong to the same linkage group. The strong sterility barriers which prevent practically all backcrosses between the hybrid and the parental species were shown to be independent of the factors regulating stylar incompatibility. L. peruvianum is heterozygous for the sterility genes which prevent fertilization or embryo formation when the interspecific hybrid is crossed, as pistillate parent, to different accessions of L. peruvianum. One peruvianum stock was found which, as a pollinator, was highly cross-fertile with the hybrids.The presence of a concentric endoplasmic reticulum in inhibited pollen tubes was observed to be a constant feature of both the self- and the unilateral incompatibility reactions and was interpreted as an indication that incompatibility might lead to a general cessation of protein synthesis. Although incompatible tubes very much resemble, in this respect, the pollen tubes cultured in vitro, it seems probable, on theoretical grounds, that the inhibition of pollen tubes in incompatible styles does not result from an absence of growth promoting substances but from the presence of a metabolic inhibitor.

The generation of new S alleles at the incompatibility locus of Lycopersicum peruvianum Mill.

SummaryA detailed analysis has been made of S genotypes in progenies derived from induced and spontaneous inbreeding processes in a clonal population of Lycopersicum peruvianum Mill. The results indicate that, in certain genetic backgrounds, induced inbreeding leads to the generation of a new S allele which usually first appears in the pistil of individuals otherwise homozygous for one of the parental specificities. When the change in specificity occurs in S heterozygotes, spontaneous self-compatibility is promoted and the new allele can be transmitted, via selfing, to the following generation.The factors and mechanisms which may be involved in the generation of new specificities at the S locus of higher plants are discussed and preliminary evidence is provided which suggests that the hypothesis of mutation by equal crossing-over is not applicable to the present study.ZusammenfassungEs wurde eine ausführliche Analyse der S-Genotypen in Nachkommenschaften einer geklonten Population von Lycopersicon peruvianum Mill, nach induzierter und spontaner Inzucht gemacht.Die Ergebnisse deuten an, daβ induzierte Inzucht bei einem bestimmten genotypischen Milieu zum Entstehen eines neuen S-Allels führt. Dieses erscheint gewöhnlich zuerst in den Griffeln einzelner Individuen, die im übrigen für eines der elterlichen Allele homozygot sind. Wenn die Änderung in S-Heterozygoten auftritt, dann wird die spontane Selbstkompatibilität gefördert. Das neue Allel kann durch Selbstung in die nächste Generation übertragen werden.Die Faktoren und Mechanismen, die am Entstehen neuer Allele am S-Locus höherer Pflanzen beteiligt Scin können, werden diskutiert. Ein vorläufiger Hinweis wird dafür erbracht, daβ die Hypothese der Mutation durch equal crossing-over für die vorliegende Untersuchung nicht zutrifft.

Radiation effects on the one locus-gametophytic system of self-incompatibility in higher plants : A review.

SummaryA brief review is presented of the temporary and permanent effects which are usually observed after irradiation treatment of self-incompatible plants with a one locus-system of gametophytic incompatibility and attempts are made to demonstrate that the study and exploitation of such effects can lead to important advances in the fields of mutation breeding, radiobiology per se and biochemistry.The fact that irradiation treatment has only negative effects on the self-incompatibility system (inactivation of the incompatibility reaction and/or genetic losses at the S locus) is discussed in relation to the positive changes (formation of new specificities) which are observed during inbreeding. In this connection, the suggestion is made that the self-incompatible plant may perhaps be equipped with a switch system or a mutagenic mechanism which enables it to display a new specificity when this new specificity is needed for increasing the level of genetic polymorphism in the population to which the plant belongs.ZusammenfassungEs wird ein Überblick gegeben über die temporären und permanenten Nachwirkungen einer Strahlenbehandlung von selbstinkompatiblen Pflanzen mit gametophytischer Determination. Die Untersuchung und Auswertung solcher Effekte kann zu Fortschritten auf dem Gebiete der Mutationszüchtung, der allgemeinen Strahlenbiologie und der Biochemie beitragen.Die Tatsache, daß die Strahlenbehandlung lediglich negative Wirkungen hat (Inaktivierung der Inkompatibilitätsreaktion und/oder genetische Störungen am S-Locus), wird vergleichend diskutiert mit den positiven Änderungen (Auftreten neuer Spezifizitäten) bei anschließender Inzucht.Es wird in diesem Zusammenhang vorgeschlagen anzunehmen, daß die selbstinkompatible Pflanze möglicherweise mit einem Schalt-System oder einem mutagenen Mechanismus ausgerüstet ist, die sie in die Lage versetzen, eine neue Spezifizität zu entwickeln, wenn diese zur Erhöhung des Niveaus des genetischen Polymorphismus in der Population notwendig ist.

Detection and cytological analysis of tomato haploids

Using the recessive character technique of detection, two haploid plants ofLycopersicon esculentum were selected among the progeny of a cross between non-irradiated individuals, homozygous recessive for three seedling markers, and wild type pollen previously exposed to thermal neutrons.The haploid plants were found to constitute a very suitable material for detailed cytological investigation. Evidence was obtained that the haploid chromosomes display in meiosis at least some degree of segmental homology during pachytene and first metaphase. This finding, which was confirmed by an analysis of distribution frequencies in anaphase, may shed new light on the speculations concerning basic chromosome number in the genusLycopersicon.The presence of a nucleolus at each of the two poles in prophase II is reported, and interpreted as a proof that other chromosomes can assume in haploids the specific function of the nucleolar organizer when the latter is absent.

The combined use of leaf irradiation and of the adventitious bud technique for inducing and detecting polyploidy, marker mutations and self-compatibility in clonal populations of Nicotiana alataLink and Otto

SummaryThe method of vegetative propagation by adventitious bud formation on detached leaves has been used, in combination with radiation treatment, for obtaining self-compatibility mutations and marker mutations in two clonal populations derived from single S2S3plants of Nicotiana alata. Out of in all 345 individuals arisen from leaves exposed to various dosages of X-rays, 3 self-compatible and 3 marker mutants were selected, which maintain their phenotypes after several cycles of vegetative propagation. These results demonstrate that it is possible to recover, in a self-incompatible clone, mutant individuals which are characterized by the same genetic background as the mother plant and which are, consequently, free of the inbreeding effects usually resulting from the conventional methods of mutation induction and detection.In addition to these six mutants, a number of plants originated from both control and irradiated leaves were found to be tetraploid and self-compatible. Whereas the occurrence of tetraploidy is taken as an indication that some of the initial cells from which the plantlets originated were polyploid, the fact that no cytochimeras were detected, is considered to present a further demonstration of the ‘one cell-one plant’ theory. Self-compatibility in the tetraploid plants is ascribed to a competition effect in heterogenic pollen.Attempts are now in progress to obtain more mutant individuals and to use the marker-mutations available for mapping the S-locus. A biochemical comparison of self-compatible and self-incompatible plants with almost identical genetic backgrounds will be initiated.

The effects of different mutagens on self-incompatibility in Nicotiana alata link and otto—I. Chronic gamma irradiation*

An analysis was made of the efficiency of chronic gamma irradiation for inducing mutations at the S-locus of the self-incompatible species Nicotiana alata . A clonal population derived from a single S 2 S 3 individual was exposed, at the flowering stage, to different dose rates of gamma rays ranging from 0·3 to 14·3 rad/hr. With a daily interruption of 3 hr, irradiation was continuous and lasted 3 months. In all more than 25 million pollen grains were screened, via repeated self-pollination in the irradiated populations for either self-compatibility mutations or new specificities. Whereas the results obtained clearly indicated that chronic irradiation was very effective for inducing seed set upon selfing, only one unequivocal mutant could be recorded among the M 2 progenies. This mutant produced compatible pollen and clearly displayed a centric fragment. Not a single case of a new specificity at the S-locus could be detected. These results are compared to those that have been obtained elsewhere after acute exposure to X-rays and the conclusion is reached that chronic irradiation is far less effective than acute treatment for inducing self-compatibility mutations. The apparent incapacity of both chronic and acute irradiation to generate new self-incompatibility alleles is discussed in relation to the spontaneous occurrence of such mutations in inbred populations.

The effects of different mutagens on self-incompatibility in Nicotiana alata Link and Otto: II. Acute irradiations with X-rays and fast neutrons

SummaryPollen mother cells of Nicotiana alata were exposed to acute doses of X-rays or fast neutrons and, at anthesis, the pollen produced was used to pollinate unirradiated plants of the same clone. The results indicate that fast neutrons are slightly more effective than X-rays for inducing self-compatible pollen-part mutations but that neither fast neutrons nor X-rays have the capacity to generate new self-incompatibility alleles. An unexpectedly high fraction of the self-compatibility mutations induced does not display the centric fragment which usually characterises such mutations. The ratio of fragment to non-fragment self-compatibility mutations of fast neutrons is identical to that of X-rays. The possible use of permanent self-compatibility mutations for production of F1 hybrid seed is discussed.SummaryPollen mother cells of Nicotiana alata were exposed to acute doses of X-rays or fast neutrons and, at anthesis, the pollen produced was used to pollinate unirradiated plants of the same clone. The results indicate that fast neutrons are slightly more effective than X-rays for inducing self-compatible pollen-part mutations but that neither fast neutrons nor X-rays have the capacity to generate new self-incompatibility alleles. An unexpectedly high fraction of the self-compatibility mutations induced does not display the centric fragment which usually characterises such mutations. The ratio of fragment to non-fragment self-compatibility mutations of fast neutrons is identical to that of X-rays. The possible use of permanent self-compatibility mutations for production of F1 hybrid seed is discussed.

Effects of chronic irradiation upon a self-incompatible clone ofLycopersicum peruvianum.

SummaryFlovering plants from a self-incompatible clone ofLycopersicum peruvianum were exposed during 90 days to different dose-rates of gamma-rays ranging from 2 to 17 rad perhour. Irradiation was continuous, except for a daily interruption of 7 hours.Whereas irradiation at dose-rates higher than 7.50 rad per hour seriously inhibited bud formation and floral development, chronic exposure at dose-rates ranging from 3 to 7 rad/hour increased the number of seeds per plant. This stimulation is not due to a higher number of seeds per fruit but to a very significant rise in the number of fruits per plant. As irradiation treatment did not significantly increase the number of flowers per plant and did not appear to stimulate pollen tube germination in the styles, it is concluded that the recorded increase in fruit-setting essentially resulted from a radio-induced inhibition of the processes which control floral abscission in the absence of cross-pollination.Three plants with reduced self-incompatibility and one completely parthenocarpic individual were observed in theM2 progeny but no evidence was obtained that irradiation could induce a permanent type of self-compatibility inL. peruvianum.ZusammenfassungBlühende Pflanzen eines selbstunverträglichen Klons vonLycopersicum peruvianum wurden 90 Tage hindurch verschiedenen Dosen von Gammastrahlen ausgesetzt (2 bis 17 rad/h). Die Bestrahlungsdauer betrug täglich seiben Stunden.Während Bestrahlungen in Dosierungen von mehr als 7,5 rad/h die Knospenbildung und Blütenentwicklung stark hemmten, steigerte eine Dauerbestrahlung mit 3 bis 7 rad/h die Samenzahl je Pflanze. Diese Erscheinung beruht nicht auf einer Zunahme der Samenzahl der einzelnen Früchte, sondern auf einer hochsignifikanten Steigerung der Fruchtzahl der einzelnen Pflanzen. Da durch die Behandlung die Blütenzahl nicht signifikant zunimmt und das Einwachsen des Pollens in den Griffel nicht gefördert wird, ist anzunehmen, daß die festgestellte Zunahme des Fruchtansatzes im wesentlichen auf eine strahleninduzierte Hemmung des Blütenverlustes bei Ausbleiben der Fremdbefruchtung zurückzuführen ist.Drei Pflanzen mit herabgesetzter Selbstunverträglichkeit und ein vollständig parthenokarpes Exemplar wurden in derM2-Generation angetroffen; es wurde kein Grund für die Annahme gefunden, die Bestrahlung könne eine erbliche Selbstverträglichkeit beiLycopersicum peruvianum hervorrufen.

BORON DEPENDENCE OF THE GENETIC RADIOSENSITIVITY DURING THERMAL NEUTRON IRRADIATION.

Abstract A modification of the ratio between the two stable isotopes 10B and 11B present in a material exposed to thermal neutrons entails a change in the total dose received, as a consequence of the capture reaction 10B (n, α)7Li. The genetic effects of this dose change were measured in the F1 progeny of a cross between two Lycopersicon species which differ for the whole genotype including specific marker genes. Untreated plants, homozygous recessive for these genes, were pollinated with irradiated dry pollen which had been collected from dominant individuals cultivated with nutrient solutions containing respectively 91% and 5% 10B. For eleven criteria, the magnitude of the genetic effects (probably deletions) induced by irradiation was found to be modified by the boron treatment in a proportion which ranged from 4 to 65%, depending upon the criterion considered. The mutation frequency at specific nuclear sites was less boron-dependent than the frequency of non-specific mutations. This finding is in agreement with the hypothesis that boron is preferentially located at some distance from the cell nucleus.

Facts and Hypotheses on the Origin of S Mutations and on the Function of the S Gene in Nicotiana alata and Lycopersicum peruvianum

Fragments in certain pollen-part mutants are suspected to originate from duplications of satellited regions. Linkage tests demonstrate that, if this interpretation is correct, either the specificity segment of the S locus is not on the fragment or two unliked specificity segments operate respectively in pollen and style. As electron microscopy and the effects of giberellic acid on selfing suggest that one manifestation of incompatibility reactions is a cessation of protein synthesis in pollen tubes, it is possible that the fragment does not carry an S locus but ribosomal cistrons necessary for the maintenance of activity. Ancestry tests are under way for testing the hypothesis that different specificity segments are organized as tandem repeats and can be switched on and off in inbred backgrounds. Electron microscopy analyses of self-incompatible hybrids indicate that a single gametophytic factor governs interspecific incompatibility in hybrid pollen and is allelic or linked to the S locus.