CYTOGENETIC STUDIES IN CLARKIA,SECTION PRIMIGENIA. I. A CYTOLOGICAL SURVEY OF CLARKIA AMOENA

Snow , Richard . (U. California, Davis.) Cytogenetic studies in Clarkia, section Primigenia. I. A cytological survey of Clarkia amoena. Amer. Jour. Bot. 50(4): 337–348. Illus. 1963.—Clarkia amoena (2n = 14) shows great cytological diversity based on reciprocal translocations. About ½ the plants sampled were heterozygotes; in 2 of the subspecies the frequency was as high as ⅔. The metaphase configurations were varied, including rings of 4, 6, 8, 4 + 4, 6 + 4, and more unusual types forming a chain of 3 + a univalent or a chain of 4 + a univalent. The latter have only 13 chromosomes but are not true monosomies. Some plants with supernumerary chromosomes were found, in addition to a plant with only 12 chromosomes which was derived from a “monosomic” type. From crosses of various homozygous lines to a standard strain, it was found that the standard end arrangement of chromosomes occurs widely and commonly throughout the species. A minimum of 7 different end arrangements, giving a ring of 4 with the standard strain, and 1 giving a ring of 6, account for the chromosome pairing found in interstrain crosses. No doubt more would be found with further testing. Some of the translocations may be components of adaptive polymorphic systems, though there is no proof of this yet. No balanced lethal systems similar to those in the closely related genus Oenothera were found.     

CYTOGENETIC STUDIES IN CLARKIA,SECTION PRIMIGENIA. II. A CYTOLOGICAL SURVEY OF CLARKIA ARCUATA AND CLARKIA LASSENENSIS

Clarkia arcuata and C. lassenensis are the 2 members of the subsection Flexicaules. Although closely related morphologically, they show very different patterns of chromosomal variability in nature. About 25% of the plants grown from wild seed of C. arcuata, a predominantly cross-pollinating species, were heterozygous for 1 or 2 translocations; such heterozygotes were found in 5 of the 9 populations sampled. An analysis of the chromosome pairing in intraspecific crosses indicated that at least 5 different translocations giving a ring of 4 with the “standard” strain, 2 giving a ring of 6, and 2 giving a ring of 8 are present in nature. No arrangement was found with widespread distribution, and it is impossible to say at present what might be the primitive arrangement of this species. One population was found to contain an inversion, a rearrangement which is very rare in Clarkia at the intraspecific level. In C. lassenensis, a predominantly self-pollinating species, only 6% (3 plants) of a sample of 53 were translocation heterozygotes, and these heterozygotes were found in only 2 of 13 populations. Intraspecific crosses indicated that one chromosome arrangement, the “standard,” was present throughout the species range.     

A CYTOGENETIC STUDY OF CLARKIA UNGUICULATA II. SUPERNUMERARY CHROMOSOMES

Mooring , John S. (Washington State U., Pullman.) A cytogenetic study of Clarkia unguiculata. II. Supernumerary chromosomes. Amer. Jour. Bot. 47(10): 847–854. Illus. 1960.—Supernumerary chromosomes morphologically comparable to those of the basic complement occur in high frequency in wild populations of this onagraceous annual plant. This paper evaluates the role that these supernumerary chromosomes play in the population dynamics of this species. The results of crossing plants with various numbers of supernumeraries showed that these chromosomes often increased in number in the seed parent. Examination of microsporocytes showed that almost 30% of the 491 plants determined had 1 or more supernumerary chromosomes. Studies of natural populations revealed that the frequency of individuals with supernumeraries varied from 0 to 79% in different populations, and also that in 1 population their frequency varied significantly in different years. It is concluded that supernumerary chromosomes in this species are adaptive under at least certain conditions. It is speculated that they are components of polygenic systems which increase tolerance to environmental extremes.     

A CYTOGENETIC STUDY OF A THREE-PAIRED RACE OF HAPLOPAPPUS GRACILIS

Haplopappus gracilis (Nutt.) Gray is an aneuploid species of Compositae of wide distribution in the southwestern U.S.A. and northern Mexico. Except for 2 types of supernumeraries, cytological samples from throughout its range have regularly shown 2 pairs of chromosomes. However, recent collections from a small area in south-central Arizona had 2n = 4, 2n = 5, and 2n = 6 in the same population. The 2n = 5 plants were hybrids between the 2n = 4 and 2n = 6 types. Both natural and artificial F₁ hybrids show preferential disjunction from a trivalent and produce genetically balanced gametes with n = 2 and n = 3. In backcrosses of the F₁ to n = 2 plants, a 1:1 ratio of 2n = 4 and 2n = 5 plants was obtained. The possible mechanisms for the evolution of n = 2 from n = 3 and vice versa are discussed.     

EVOLUTIONARY MODIFICATION IN CLARKIA. II. ROLE OF FLOWER BUD PUBESCENCE IN SECTION PHAEOSTOMA

Observations of the habitats and relative flowering of a Clarkia species with hairy flower buds and several with hairless flower buds led to the hypothesis that long hairs on flower buds regulate bud temperature. This hypothesis predicts that hairless buds would be warmer and develop faster than hairy buds, which would be cooler, develop more slowly, and avoid high temperature stress. The hypothesis was tested by comparing flower bud growth rates and temperatures in three genetically similar biotypes of Clarkia unguiculata and in all six species of section Phaeostoma. Flower buds of the three biotypes included hairy (HY) and hairless (HN) from the same coastal population and densely hairy (HD) from an interior locality. The six species included C. unguiculata with densely hairy buds (HD) and five related species with hairless buds. Contrary to expectations, HY buds grew more rapidly than HN buds. HD buds grew more rapidly than either and also more rapidly than the hairless buds of five related species. Again contrary to expectations, the three biotypes of C. unguiculata had equivalent temperature relations, with bud temperatures mostly somewhat below air temperatures. In a comparative experiment, bud temperatures in C. unguiculata approximated air temperatures while bud temperatures in five related species mostly fell well below air temperatures. Thus, predictions of the hypothesis were not borne out. Long bud hairs apparently have minimal effect on bud growth rates and temperatures, and we conclude that physiological adaptations are more important. Bud cooling mechanisms are discussed.     

糜子双受精过程的细胞学观察

糜子(Panicum miliaceum L.)受精的全过程在开花后3小时内完成。开花后20分钟,花粉管到达珠孔,30分钟进入胚囊并释放精子;雌、雄性核融合发生在开花后30分钟至3小时。精核与卵核和极核融合的过程基本相同,但总是先完成与极核的融合。开花后2小时,初生胚乳核形成,随后立即分裂。开花后3小时,合子形成,此时胚乳含两个游离核。开花后8—10小时,合子进入分裂期。合子的休眠期约5—7小时。受精作用属于有丝分裂前配子融合的类型。     

CYTOLOGICAL STUDIES OF OEDOGONIUM. I. OOSPORE GERMINATION IN O. FOVEOLATUM

Details of oospore germination, including meiosis, are described and illustrated as they appear in the homothallic Oedogonium foveolatum. Unequivocal meiotic division is demonstrated to occur within 12 hr prior to the liberation of the motile tetrad cells. The development and liberation of the meiotic products are described as are instances of anomalous oospore germination. In some cases, meiosis apparently fails to occur, and a single, diploid germling invariably results. Various factors influencing oospore germination are discussed, including an apparent absolute light requirement.     

CYTOLOGICAL STUDIES OF OEDOGONIUM. II. CHROMOSOME NUMBERS IN TWELVE SPECIES OF OEDOGONIUM

Mitotic chromosome counts are reported for 12 species of Oedogonium with the following distribution: two species with 13 chromosomes, two species with 16 chromosomes, five species with 17 chromosomes one species with 19 chromosomes, one species with 32 chromosomes, and one species with 38 + 1 chromosomes. Diploid strains of two species are illustrated and discussed. Cytological comparison of species establishes that there is great diversity in Oedogonium with relation to chromosome number, size, and morphology.