Gametic Lethals Research Articles

Toward the identification of dwarfing gene d60 that complements gametic lethal gene gal by Next-Generation Sequencer oriented genome analysis

The author previously discovered dwarfing gene, d60, and it complements the gametic lethal gene, gal, to cause gametic lethality in rice. Namely, in the F1 hybrid (genotype D60d60Galgal) of Koshihikari (D60D60galgal) × Hokuriku 100 (d60d60GalGal), male and female gametes having both gal and d60 become gametic lethal and the pollen and seed fertility decrease to 75%. Therfore, F2 progeny shows a unique mode of inheritance that is segregated into a ratio of 6 fertile long-culm (4D60D60: 2D60d60GalGal) : 2 partially fertile long-culm (D60d60Galgal = F1 type) : 1 dwarf (d60d60GalGal).Prior to Next-Generation Sequencing analysis targeting d60, it is required to develop isogenic genome of reference except for target d60 gene. When, the F1 (D60d60Galgal) progenies of ‘Koshihikari’ × ‘Hokuriku 100’ were backcrossed to ‘Koshihikari’ (D60D60galgal), BC1 F1 individuals segregated in a ratio of 1 tall and 25% sterile (D60d60galGal):2 tall (1 D60D60Galgal:1 D60D60galgal). Here, D60d60Galgal-heterozygous plants, can be recognized by pollen sterility prior to anthesis. Then Tall and 25% sterile BC1F1 plants (D60d60galGal) were selected by 25% pollen sterility and backcrossed with ‘Koshihikari’ as female parent to produce BC2F1 seeds. Hereafter, D60d60Galgal-heterozygous plants were selected in the first generation of each backcross (BCnF1) and immediately backcrossed repeatedly with Koshihikari to obtain isogenic genome useful for next-generation sequencing. That is to say, the target d60 DNA region narrowed down with each back cross generation, enabling identification of the target DNA mutation point by whole genomic sequencing.

An evolutionarily conserved mitochondrial orf108 is associated with cytoplasmic male sterility in different alloplasmic lines of Brassica juncea and induces male sterility in transgenic Arabidopsis thaliana

Nuclear-mitochondrial gene interactions governing cytoplasmic male sterility (CMS) in angiosperms have been found to be unique to each system. Fertility restoration of three diverse alloplasmic CMS lines of Brassica juncea by a line carrying the fertility-restorer gene introgressed from Moricandia arvensis prompted this investigation to examine the molecular basis of CMS in these lines. Since previous studies had found altered atpA transcription associated with CMS in these lines, the atpA genes and transcripts of CMS, fertility-restored, and euplasmic lines were cloned and compared. atpA coding and downstream sequences were conserved among CMS and euplasmic lines but major differences were found in the 5' flanking sequences of atpA. A unique open reading frame (ORF), orf108, co-transcribed with atpA, was found in male sterile flowers of CMS lines carrying mitochondrial genomes of Diplotaxis berthautii, D. catholica, or D. erucoides. In presence of the restorer gene, the bicistronic orf108-atpA transcript was cleaved within orf108 to yield a monocistronic atpA transcript. Transgenic expression of orf108 with anther-specific Atprx18 promoter in Arabidopsis thaliana gave 50% pollen sterility, indicating that Orf108 is lethal at the gametophytic stage. Further, lack of transmission of orf108 to the progeny showed for the first time that mitochondrial ORFs could also cause female sterility. orf108 was found to be widely distributed among wild relatives of Brassica, indicating its ancient origin. This is the first report that shows that CMS lines of different origin and morphology could share common molecular basis. The gametic lethality of Orf108 offers a novel opportunity for transgene containment.

Determination of the probable ontogenic time of action of lethal factors on thesecond chromosome of Drosophila melanogaster derived from a natural population

From the study of the genetic load of second chromosome factors in a natural population of Drosophila melanogaster, 15 lethal-bearing strains were recovered and maintained in the laboratory balanced against Ins (2L+2R), Cy, L4. For each lethal factor, the probable time of action during development was determined by the appearance of a sharp reduction, at any given stage, in the frequency of individuals compared to that expected in the absence of the lethal factor. Carried out in this way, the analysis suggested that sevne were embryonic lethals, two larval lethals and three pupal lethals. Additionally, three gave no evidence of affecting any of the above-mentioned stages; these are interpreted as gametic lethals.

X-4 Translocations and Meiotic Drive in Drosophila melanogaster Males: Role of Sex Chromosome Pairing

Males carrying certain X-4 translocations exhibit strongly skewed sperm recovery ratios. The X(P)4(D) half of the translocation disjoins regularly from the Y chromosome and the 4(P)X(D) half disjoins regularly from the normal 4. Yet the smaller member of each bivalent is recovered in excess of its pairing partner, apparently due to differential gametic lethality. Chromosome recovery probabilities are multiplicative; the viability of each genotype is the product of the recovery probability of its component chromosomes. Meiotic drive can also be caused by deficiency for X heterochromatin. In(1)sc(4L)sc(8R) males show the same size dependent chromosome recoveries and multiplicative recovery probabilities found in T(1;4)B(S) males. Meiotic drive in In(1)sc(4L)sc(8R) males has been shown to be due to X-Y pairing failure. Although pairing is regular in the T(X;4) males, the striking phenotypic parallels suggest a common explanation. The experiments described below show that the two phenomena are, in fact, one and the same. X-4 translocations are shown to have the same effect on recovery of independently assorting chromosomes as does In(1)sc(4L)sc(8R). Addition of pairing sites to the 4(P)X(D) half of the translocation eliminates drive. A common explanation-failure of the distal euchromatic portion of the X chromosome to participate in X:Y meiotic pairing-is suggested as the cause for drive. The effect of X chromosome breakpoint on X-4 translocation induced meiotic drive is investigated. It is found that translocations with breakpoints distal to 13C on the salivary map do not cause drive while translocations broken proximal to 13C cause drive. The level of drive is related to the position of the breakpoint-the more proximal the breakpoint the greater the drive.

SEX CHROMOSOME MEIOTIC DRIVE IN DROSOPHILA MELANOGASTER MALES

In Drosophila melanogaster males, deficiency for X heterochromatin causes high X-Y nondisjunction and skewed sex chromosome segregation ratios (meiotic drive). Y and XY classes are recovered poorly because of sperm dysfunction. In this study it was found that X heterochromatic deficiencies disrupt recovery not only of the Y chromosome but also of the X and autosomes, that both heterochromatic and euchromatic regions of chromosomes are affected and that the "sensitivity" of a chromosome to meiotic drive is a function of its length. Two models to explain these results are considered. One is a competitive model that proposes that all chromosomes must compete for a scarce chromosome-binding material in Xh(-) males. The failure to observe competitive interactions among chromosome recovery probabilities rules out this model. The second is a pairing model which holds that normal spermiogenesis requires X-Y pairing at special heterochromatic pairing sites. Unsaturated pairing sites become gametic lethals. This model fails to account for autosomal sensitivity to meiotic drive. It is also contradicted by evidence that saturation of Y-pairing sites fails to suppress meiotic drive in Xh(- ) males and that extra X-pairing sites in an otherwise normal male do not induce drive. It is argued that meiotic drive results from separation of X euchromatin from X heterochromatin.

ANALYSIS OF GENES CONTROLLING F1 STERILITY IN RICE BY THE USE OF ISOGENIC LINES

ABSTRACT In order to look into the genetic basis of intervarietal F1 sterility in rice (Oryza sativa L.), a series of backcrosses (up to B13) was carried out using Taichung 65 (Japonica type) as the recurrent parent and several Indica varieties as donor parents. A number of "isogenic F1-sterile lines" were isolated by test-crossing fertile F2 plants obtained from the selfing of partly pollensterile backcross segregants. Crossing experiments with the isogenic lines confirmed the author's previous hypothesis that there are sets of duplicate gametic lethals (s genes) and that gametes carrying a double recessive combination (s1s2) of these deteriorate during development, though in the present hypothesis the genes are considered to affect the development of microspores only. Assuming that Taichung 65 has the genotype s1/s1 + 2/+2 and a donor parent (like an isogenic F1-sterile line derived from it) has +1/+1s2/s2, pollen grains with +1s2 have shown a higher fertilizing capacity in the genetic background of Taichung 65 than those with s1 +2, while those with +1 +2 have a lower fertilizing capacity. This certational advantage of alien genes was considered to be an internal mechanism that helped the development of F1 sterility relationships among rice varieties. The isogenic F1-sterile lines derived from different donor parents each had a set of s genes at different loci. Linkage relations were detected between the s loci and three gene markers.

Kopplungsanalysen zur Lokalisation einiger häufig mutierender Loci bei Antirrhinum majus L.

1. Three new spontaneous mutations of the caeca-type (reduction of yellow flower pigments) were found to be homologous, but are not alleles of the authentic caeca locus. However, two new graminifolia mutations are alleles of the authentic gram locus. 2. Six loci, characterized by a high frequency of forward mutation, were studied, three are located on the gram-chromosome. This chromosome thus contains six unstable loci: graminifolia, incolorata-II, incolorata-III, silvescens-II, caeca-I and fimbriata. 3. The gene-sequence of the gram-linkage group is: del - inc.-III - gram - silv.-II - cen - inc.-II - cae.-I - fim. 4. The loci of incolorata-IV and silvescens-I were localized in the uni-chromosome. Both of them occasionally exhibit greatly increased crossing-over exceeding the 50 percent limit. It is assumed that compensated gametic lethals are the cause of this phenomenon.

INSEMINATION AND SPERM STORAGE INDROSOPHILA MELANOGASTER

The assumption that the four products of meiosis in the male Drosophila are equally functional has been made untenable by recent studies of abnormal progeny ratios. The potential causes of anomalous ratios can be classified in three categories: (1) postfertilization selection; (2) aberrant meiotic segregations; and (3) gametic lethality or selection. Unusual ratios resulting from zygotic lethality are well known, and it is relatively easy to fix this as the cause of an abnormal ratio by comparing the number of eggs laid to the number of adults recovered. There are also deviations from expected ratios which have been attributed to phenomena occurring during meiosis which render some of the products of meiosis non-functional (Novitski and Sandler, 1957; Sandler, Hiraizumi, and Sandler, 1959). Gametic lethality has also been implicated as a possible cause of anomalous ratios (C. W. Metz, 1929; Gershenson, 1928; Novitski, 1947). However, in these studies where gametic lethality is suggested as a possible cause of abnormal progeny ratios, this has been as a result of eliminating other possibilities and no mechanism has been described to account for the phenomenon. In fact, it has been generally held that the DNA of the spermatozoon is wholly inert and that the functional ability of the spermatozoon is independent of its gene content (Muller and Settles, 1927). Recent studies of sperm DNA suggest, however, that there may be changes in the

Cytogenetic Evidence for the Inclusion of <i>Oenothera affinis</i> Camb. under <i>Oenothera mollissima</i> L

The cytogenetic behavior of 66 hybrids between Oenothera affinis and Oenothera mollissima has been described.Races Carrasco, Buenos Aires, and Argentina of O. mollissima have been shown to be “heterogamous” as pollen parent, but “isogamous” as seed parent. Races Toledo, Florida and Palermo of O. affinis proved to be isogamous as either parent. Evidence concerning the behavior of gametic lethals is less certain for the other races of O. mollissima, but they too probably follow the pattern presented above. Evidence has also been presented for the occurrence of various types of zygotic lethals.Hybrids which could be classified as one or the other of the two species have been discussed. The phylogenetic relationships between the various races of O. affinis and O. mollissima have been pointed out. The various races of O. mollissima were found to be more distantly related to one another with respect to segmental interchanges than they were to the races of O. affinis. The cytogenetic evidence, thus, favors a consolidation of O. affinis and O. mollissima under a single species.

Pollen germination in stocks and the possibility of applying a lethal factor hypothesis to the interpretation of their breeding

(1) Tests have been carried out of the germination on artificial media of the pollen of double-throwing races (d. races) and pure breeding single races (no-d. races) of Stocks (Matthiola incana M. Br.). The percentage of germination obtained in the no-d. races ranged from 76 to 29, and that in the d. races from 29 to 1.5. The difference is statistically significant. (2) The evidence of the germination tests can, perhaps, be accepted as sufficient to show that a difference exists between the pollen of the d. and no-d. races in respect to the possibility of germination. If this is so, theories which attempt to explain the genetical behaviour of Stocks in terms of pre-maturation segregation lose their main support. (3) Rival theories, such as those which postulate gametic lethals, must therefore be considered. A theory of this type is brought forward, which is claimed to account for the facts better than previous similar theories: but it does not fit the facts any better than does Saunders’ scheme, which, however, does not account for the lack of certain classes of gametes. (4) It is shown that the hypothesis brought forward in this paper provides a plausible explanation of the breeding results which have been obtained by Frost, working with the trisomic race Slender.