AN ALBINO MUTANT IN P L A N T A G O ZNSULARZS REQUIRING THIAMINE PYROPHOSPHATE. I: GENETICS
SANDRA M. MURR2 AND G. LEDYARD STEBBINS Department of Genetics, Uniuersity of California, Dauis, Calif. 95616
Received October 5, 1970
ALBINO mutants are one of the most common hereditary defects resulting from mutagens such as gamma rays. Most of the mutations induced by ioniz- ing radiations in higher plants involve single recessive genes but even these do not always segregate in monogenic ratios (MOH and SMITH 1951). A recessive albino mutant requiring thiamine pyrophosphate has been induced by gamma irradia- tion of the seed in Plantago insularis. Heterozygotes for this mutant give a segre- gation ratio of 1 normal to 1 mutant when they are selfed. The purpose of this study was to clarify the genetics of this apparently aberrant segregation. The involvement of thiamine pyrophosphate in the physiology of the mutant and the effects of its deficiency on the ultrastructure of chloroplast development will be considered in later publications.
MATERIALS A N D METHODS
Seeds were sterilized by immersion in a 1 : 1 solution of absolute ethanol and 6% hydrogen peroxide (after LANGRIDGE 1957) for 5 min, then rinsed in sterile distilled water. They were germinated at 27°C under continuous light for 12 hr in a solution containing 50 ppm potassium gibberellate (85%) and 0.4% sucrose. The seedlings with radicles from 1 to 2 mm long were transferred with a Pasteur pipette to flats containing fine sand. The flats were watered at weekly intervals with a nutrient solution of JOHNSON et d . (1957) as modified by STEBBINS and DAY
(1967). To prevent moisture loss the flats were covered with transparent plastic for 3 days or until the cotyledons were fully emerged from their seed coats. Plants were maintained in a growth chamber on a 12 hr light regime of 1500 ft-c of mixed incandescent and fluorescent lamps with temperatures of 27°C during the light and 18°C during darkness.
Slides for pachytene analysis of pollen mother cells (PMC's), mitotic analysis of root tips, and pollen viability were prepared according to STEBBINS and DAY (1967) and WHITTINGHAM and
STEBBINS (1969). The thiamine pyrophosphate requirement for greening and normal growth was not recognized until after the genetic analysis was started. Since the mutant did not survive to flowering, plants heterozygous for the mutant character were crossed with homozygous trans- location lines to localize the mutant gene. Two somatically distinct homoszygous translocation lines were used: T3-122.3 involving chromosomes 3 and 4 and T4-73.41 involving chromosomes
1 and 4 .
Characterization of the mutant: The albino mutant of Plantago insularis
(2n = 8 ) was discovered among the X, progeny of a y-irradiated seed (WHITT-
Frum a thesis submitted by S.M.M. in partial fulfillment of the requirements for the Degree of Doctor of Philosophy
Present address: Dept. of Animal Science, University of California, Davis. Calif. 95616.
in Genetics. This investigation was supported by NIH Training Grant GlM701 and in part by NSF grant GB5713X.
232 S. M. MURR AND G. L. STEBBINS
INCHAM and STEBBINS 1969). The mutants were completely white and survived
only until the 13th day from planting when grown under normal greenhouse conditions. The characteristics of the standard pachytene karyotype (Figures 1 A,
B) are discussed in detail by WHITTINGHAM and
STEBBINS( 1969). The
X Ihetero- zygote was found to have a heterozygous reciprocal translocation between chro- mosomes 3 and 4 at the centromere region (Figures lC,
3-4 I C
A N A L B I N O M U T A N T I N P L A N T A G O 233
When the albino mutants first appeared in a planting of seeds from T,-103, the
X,progeny segregated 37 green to 13 albinos closely approximating a 3 : 1 ratio as would be expected if the albino mutant were a single recessive gene. Later, however, when selfed seeds were collected from each of the
5branches of the XI plant, only branches 1 and 2 segregated both green and albino offspring in a 1 : 1 ratio, while branches 3,4, and 5 produced only green progeny (Table 1 ) . Chro- mosome analyses of PMC’s in each branch indicated that only branch 3 had the
3 ; 4 translocation. The original 3 : 1 segregation was obviously due to a mixture of seed from segregating and nonsegregating branches. The differences in the 5 branches indicate that the original plant was a chimera as a result of gamma irradiation.
Pollen viability was 50% in branches I and 2 which produced albinos, while viability in branch 3 was 68% as might be expected for a heterozygous trans- location. Branches 4 and 5 which produced only green progeny upon selfing had essentially complete pollen fertility. Cytological observations of all stages of PMC meiosis in branches 1 and 2 revealed no abnormalities in chromosome behavior. When the progeny of each of the 5 branches was selfed to the X, generation, progeny from branches 1 and 2 continued to produce green and albino plants in a
1 : 1 ratio while progeny from branches 3,4, and
5continued to yield only green plants. In all cases, the observed deviations from this ratio had a probability higher than 0.40.
In reciprocal crosses between branches 1 and 2 (Table 2 ) , the 1 : 1 ratio of
green to albino offspring was continued through Fa, indicating that the gene causing albinism is the same in both branches. The goodness-of-fit probability for a 1 : 1 ratio in these crosses is better than 0.40. The green progeny from these crosses continued to have about 50% pollen viability.
Abnormal transmission of the albino gene: Reciprocal crosses were made be- tween branch l of T,-103 and a plant giving only green progeny and having no chromosomal abnormalities. When T,-103.1 was used as the female parent, all of the F, progeny were green and half of these had 50% sterile pollen (Table 3A).
The F, progeny of plants with 50% pollen sterility were all green with half of these having 50% pollen sterility. The plants with 50% pollen sterility continued to produce in F, all green plants with half having 50% pollen sterility. Plants with full pollen fertility yielded green plants with all fertile pollen. The selfed F, plants having all fertile pollen produced 3 green : 1 albino in the F, genera- tion. All of the green F, plants had fertile pollen. In the F, generation, 1/3 of the green F, plants gave all green plants with fertile pollen, while the remaining
produced a ratio of 3 green : 1 albino with all the green plants having fertile pollen.
When T,-103.1 was used as the male parent (Table 3B) in a cross with a
A N A L B I N O M U T A N T I N PLANTAGO 235
Phenotype and pollen viability of progeny from reciprocal crmses between branches I and 2
F, F2 Fa
Crosses Percent Percent Percent
involving branches pollen pollen pollen
of 'l'*-l03 Green Albino viability Green Albino viability Green Albino viability
54 46 49.3 51 44 52.7 4 7 5 2 48.6
1 9 X 2 6
2 9 X 1 8 48 5 2 50.7 55 4.5 49.7 48 5 2 51.5
The reciprocal crosses revealed that the albino gene is transmitted through both the pistillate and the staminate parent thereby ruling out maternal inheri- tance. No appreciable female gamete abortion occurred in this or other crosses since the plants had a full seed set. Although zygotic elimination could result in defective seeds, this did not occur because all separate collections of seeds showed at least 98% germination. Since the progeny from branches 1 and 2 (Table 1) selfed to the X, generation showed a 1 : 1 ratio of viable to inviable pollen, some type of gamete elimination might be expected in this material. It has long been known that among higher plants the susceptibility of the male gamete to, abortion is determined by the genotype or by chromosomal abnormalities of the gametes. The phenotypic information in Table 3 permitted the determination of the geno- types of the plants involved in the reciprocal crosses (also Table 3 ) . If the genetic
Phenotype, genotype, and pollen viability from reciprocal crosses of T,-IOS.I
Cross Fl F2 F3
0 T3-103.1, green, ss* & I &
47 green, f
al+ pe+ 1 al+ pe+
-1100 green, f
52 green, 8s
al+ pe+ I .I+ pe 50 green, f 4 9 green, P S
d T1-122.1, green, ft
.l+ pe+ 1 a1+ pe+
48 green, f
25 green, f
al+ pe+ pe+
--99 green, f
1a1 pe+ / a1 pe+
1.I+ pe+ / a1 pe+
0 T3-122.1, green, f
.1+ pe+ I .I+ pe+
g m e t e +
236 S . M. M U R R AND G . L. STEBBINS
constitution of branches 1 and 2 were such that a recessive gene for albinism
( a l ) were linked in repulsion with the normal allele for a pollen-eliminator gene ( p e + ) , i.e., al+ pe
/al pe+, then the ratios and inheritance patterns observed would be explained.
With the heterozygous mutant as the female parent, half of the F, offspring are homozygous for the wild-type albino alleles but heterozygous for the pollen- eliminator alleles. The other half are heterozygous for the albino allele and homo- zygous for the wild-type allele of the pollen eliminator gene. The F, plants hetero- zygous for the pollen-eliminator gene when selfed continued to give progeny half heterozygous and half homozygous for the normal allele of the eliminator gene. This is caused by survival of both kinds of female gametes ( p e t and p e )
but survival of only one kind of male gamete ( p e + )
.The abortion of p e gametes would account for the 50% pollen sterility. The green plants with fertile pollen are heterozygous for the albino gene but homozygous for the normal allele of the pollen eliminator. Since the eliminator was no longer present in these crosses. a normal 3 : 1 ratio of green to albino progeny was observed from heterozygotes in the F, and F, generation.
With the heterozygous mutants in repulsion in the male parent (Table 3B), the eliminator gene is not transmitted through the male parent, so that all F, off- spring are heterozygous for albinism but homozygous wild type at the pollen- eliminator locus. A 3 : 1 ratio of green to albino progeny occurs in the F, gen- eration and the F2 heterozygotes continue to produce green and albino offspring in a 3 : 1 ratio. That does not imply that it is a recessive gene. Since the gene apparently operates in the haploid male gamete, it is not possible to use the terms recessive or dominant.
Evidence for pollen elimination: To determine whether the elimination of the male gamete carrying the al+ gene was the cause for a 1 : 1 ratio of green to albino progeny from selfed heterozygotes and not the result of nonrandom segre- gation in the formation of the female gametes, pairs of seeds (Plantago insu!aris
A N A L B I N O M U T A N T I N PLANTAGO
Frequency of phenotype classes of seed pairs from selfed heterozygous mutants
Plant Green-green Greenlalbino A1bino:albino Total Total
number pairs pairs pairs green albino
T,- 103.1 - 1 2 3 4 5 T,-I03.2-1 2 3 4 5 Total 8 7 10 12 9 11 12 10 11 10 100 18 20 18 19 20 21 15 22 18 21 192 14 13 12 9 11 8 13 8 11 9 108 34 34 38 43 38 43 39 42 40 41 392 46 46 42 37 42 37 41 38 40 39 408
likely mechanism for obtaining the observed 1 : 1 ratio of green to albino progeny.
Localizat.’on of mutant genes: Normally when mutant genes are mapped using aneuploids or translocations, the mutant is crossed in the homozygous recessive form. but since the albino seedlings do not normally survive to flowering, another method of mapping was devised using homozygous translocations. Figure 2 shows pachytene ideograms of the two homozygous translocation stocks used in locating the mutant genes. These translocation lines were chosen because they are very easy to recognize at mitosis as well as meiosis. The first translocation line, T,-
73.41 (Figure 2A) is a whole-arm translocation involving chomosomes I and 4
Phenotype and karyotype of progeny when heterozygous mutant crossed with homozygous translocation lines
Cross F1 F2
y G/ a1+ pe+ homozygous translocation
2 9 green 9 homozygous translocation
2 0 heterozygous translocation
A . X al+ pe+ / a1 pe+
11 albino 11 homozygous standard dal+ pe I a1 pe+ heterozygous &;i
translocation (Segregation type 1) standard karyotype
yal+ pe+ / al+ pe+
7 homozygous standard homozygous 2;i a1+ pe+ / a1 pe+ 27 green 11 heterozygous translocation
translocation 9 homozygous translocation
X heterozygous 1;i
translocation 4 homozygous standard
dal+ p e / a 1 pe+ 13 albinos 6 heterozygous translocations
3 homozygous translocations standard karyotype
238 S. M. MURR AND G. L. STEBBINS
to give rearrangements I s ; 4 , and 1L;4s. I n comparison with chromosomes of the standard karyotype, the Is;4L chromosome is easily recognized because it has a much smaller heterochromatic region.
The second line, T,-122.3, (Figure 2B) is a rearrangement involving the short arm of chromosome 3 and a portion of the euchromatin of the long arm of chro- mosome 4 . This exchange produces a distinctive chromosome with a heterochro- matic section on the end of the long, mostly euchromatic arm of chromosome 4 and nucleolar organizer on both ends of the chromosome. The other chromosome,
4 L ; 3 L , is very small at mitosis and has a heterochromatic region on only one side of the centromere.
Since the albino plants could not be analyzed for karyotype at pachytene, the two translocation lines were chosen because they can easily be recognized at mitosis in root tips. Lines that have a homozygous wild-type genotype and a re- arranged karyotype involving either chromosomes I and 4 or chromosomes 3 and
4 were crossed with the heterozygous mutant which has a standard karyotype (Table 5 ) . Since the heterozygous mutant was used as the male parent. all male gametes carried only the aZ pe+ genes (assuming no crossing over) making all F, off spring heterozygous for the albino alleles, homozygous for the wild-type allele of the pollen eliminator, and heterozygous for the translocation. I n the F2 gen- eration, the albino progeny may occur either with each karyotype in a ratio of 1 standard : 2 heterozygous : 1 homozygous or with only the homozygous karyo- type. If they occur with only the homozygous standard karyotype, then the gene locus is located on one of the chromosomes involved in the rearrangement (segre- gation type 1 ) . If the albino offspring occur with all three possible karyotypes,
A N A L B I N O M U T A N T I N PLANTAGO 239 then the locus is not on either of the rearranged chromosomes (segregation type When the homozygous translocation line involving chromosomes 1 and 4 is crossed to the heterozygous mutant with standard karyotype, F, segregation of type 1 is produced (Table 5A), while the cross using the homozygous transloca- tion line involving chromosomes 3 and 4 produced F, segregation of type 2 (Table 5B). These segregation patterns locate the mutant genes on chromosome 1. I n actuality this results in the localization of the albino locus since the pollen- eliminator gene is not transmitted to the F, generation. Since the alb'ino and pollen-eliminator genes are tightly linked because no crossing over has been ob- served, localization of one gene determines the location for both. The complete association of a1 with the homozygous standard karyotype suggests tight linkage with the locus of pe and the translocation breakpoint (centromere). The standard, heterozygous and homozygous translocations at metaphase in root-tip cells are shown in Figure 3.
During normal meiosis genes and chromosomes are precisely reproduced and the bivalents segregate at random to opposite poles. Among the many apparent exceptions to this 2nd Mendelian law, most do not conflict with the principle of
random chromosome segregation, but are due to postmeiotic selection. The segre- gation of genes linked to a gamete-eliminator gene will be distorted, the degree of distortion depending on the intensity of the linkage. In fact, genes affecting gametophyte development are often discovered as a result of such distortion. The present study shows that segregation of a wild allele at the albino locus is dis- torted due to complete linkage with a pollen-eliminator gene resulting in a 1 : 1 ratio of normal to mutant progeny. The data indicate linkage between a1 and pe+
with a frequency of crossing over so low that among the 3000 green plants that were grown from selfed heterozygotes and examined for pollen abortion, no homozygous green nor heterozygous green and pollen-fertile crossover individuals were found.
Since the PMC's in the heterozygous mutant undergo normal meiosis, the pe
gene apparently becomes effective after the second division of meiosis is com- pleted. The pe microspores do not increase in size and the nucleus does not divide. Instead. the microspores undergo progressive degeneration and by anthesis are virtually empty and are only 15p in diameter while viable pollen grains are 27p.
Other segregation-distortion mechanisms: Other mechanisms to explain the distorted segregations found in this study have been considered. Low germination
of the homozygous wild-type class of seeds must be excluded as a possible mecha- nism because at least 98% germination was found for all collections of seeds. Since nearly full seed set always occurred in all infloresences examined, elimina- tion of female gametes could not account for the results.
240 S. M. MURR A N D G. L. STEBBINS
FIGURE 3.--Metaphase karyotype of root-tip crlls. A: Stantlertl karyotype. D: I-ioniwmgous I ; 4 translocation (TJ3.41). C: Heterozygous I ; 4 translocation ( T , - i 3 . 4 1 ) . D: Homozygous 3;4 translocation (T3-122.3). E: Heterozygous 3;4 translocation (T:,-l22.3).
for the albina-7 factor. the xantha-3 factor. or for both, show heterosis. Although no alf/al+ homozygotes were produced from the heterozygotes in this study, heterosis W A S studied indirectly by comparing heterozygotes to unrelated wild- type plants. They were found to be the same in germination rate, size, spike num- ber. and in seed production.
Cytoplasmic male sterility, as reported in maize (RHOADES 1933). onions (JONFS and CLARKE 1943). and in sugar beets (OWEN 1945) does not seem to be a likely mechanism since this condition would be expected to affect both pollen carrying the gene for the albino character and pollen carrying the wild-type allele.
RHOADES(1942) demonstrated that an abnormal type of chromosome 10 in
A N ALBINO MUTANT I N PLANTAGO 241 the ovules received the abnormal chromosome instead of the 50% expected with random segregation. Pollen with the abnormal chromosome 10 was only partially successful in competing with pollen possessing a normal chromosome 10. Cyto- logical information concerning preferential segregation is not available in Plan- tago. It would be impossible to detect such a mechanism without heteromorphic cytological markers. There is no evidence available which would rule out this mechanism as a basis of the apparently non-Mendelian segregation, but the 50% pollen inviability makes it unlikely that this mechanism is occurring.
No meiotic abnormalities involving translocations, inversions, large duplica- tions or deficiencies, or nondisjunction were observed which could account for the observed deviations from a 3 normal : 1 albino segregation ratio. However, as pointed out by STADLER (1933), genic changes and certain chromosomal aberra- tions. especially small deficiencies, cannot always be distinguished from each other. A number of mutant characters in Drosophila, such as Notch and Minute, are known to be the result of segmental deficiencies. The condensed chromosomes of plants are not as favorable for revealing small deficiencies as are the salivary gland chromosomes of the Diptera, so it is possible that the pollen eliminator may not be a gene, but a small undetectable deficiency that causes abortion of the male gamete.
Parallel examples of segregation distortion: MOH and SMITH ( 1952) described a case in barley in which three coincidental changes occurred as a result of ex- posure to radiation from an atomic bomb. These were a factor for mutant white seedlings, a chromosomal interchange, and a factor in one of the chromosomes involved in the interchange which is transmissible through the eggs but not through the pollen. The translocated chromosome was transmitted by none of the males, and about half of the female gametes transmitted the mutated gene. In selfed progenies of the irradiated plant, 50% of the seedlings were white, and all of the green plants were heterozygous for the reciprocal translocation and both mutated genes.
Another mutant line of barley exposed to atomic bomb irradiation studied by NILAN and MOH (1955) was characterized by a high frequency of cream seed- lings arising from partially ovule-sterile, green plants. The ovule sterility of the parental plants averaged 40% and no pollen abortion was found. Following self- fertilization approximately 39% of the offspring were cream. Ovule sterility and cream seedling were found to be linked in repulsion with approximately 7.4% crossing over. Presumably, the high frequency of cream seedlings was caused by the anomalous female gametic ratios resulting from the partial ovule sterility. Progeny homozygous for ovule sterility did not appear.
HOLM (1954) reported a case in barley similar to the finding in this study. When the heterozygous chlorophyll-deficient mutant was selfed, the homozygous normal class was absent or nearly so and the heterozygotes were produced in approximately a 1 : 1 ratio with the mutant. Although the heterozygotes had a
degree of semisterility, HOLM did not hypothesize a mechanism to explain his observations.
242 S. M. MURR A N D G. L. STEBBINS
were equally affected by an eliminator gene. The gametes, however, aborted only in certain hybrid combinations. He was able to map many genes by their degree of distortion from the expected F, segregation ratio due to linkage with the gamete eliminator. The recombination distances estimated in this manner corresponded approximately with those estimated by standard methods.
The only one of these three examples that is closely similar to ours is the one described by
HOLM.The case of NILAN and MOH involves abortion only of female gametes, while in that described by RICK, both kinds of gametes aborted at ap- proximately equal frequencies. A further understanding of the similarities and differences between these examples could be obtained only after detailed develop- mental and biochemical analyses of the action of the genes involved.
technique and for making available the stocks that were used.
The authors are deeply indebted to Dr. ALVA D. WHITTINGHAM for her continuous help with
S U M M A R Y
The albino mutant al in Plantago obtained from gamma radiation acts as a recessive gene giving a segregation ratio of 1 normal to 1 albino when the hetero- zygote is selfed. Departure from a 3 : 1 ratio is attributed to complete trans
linkage with a pollen-eliminator gene ( p e )
.Half of the pollen from a l f p e ,/ al pe+ heterozygotes aborts, and the mechanism for elimination of a l f p e pollen grains is 100% effective. The albino and pollen-eliminator genes were mapped to chro- mosome I using translocation markers in somatic cells. Apparently when the seed producing the original X, plant was irradiated with gamma rays? three changes occurred: (1) a reciprocal translocation between chromosomes 3 and 4 ;
(2) an albino-causing mutation on one homologue of chromosome I ; and ( 3 ) a
mutation causing the elimination of microspores on the other homologue of chro- mosome I .
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