Facultative Apomicts Research Articles

Cytology and reproductive behavior of hybrids between Paspalum urvillei and two hexaploid P. dilatatum biotypes

Sexual, tetraploid (2n = 4x = 40) vaseygrass (Paspalum urvillei Steud.) was crossed with two apomictic, hexaploid (2n = 6x = 60) dallisgrass (P. dilatatum Poir.) biotypes (Uruguayan and Uruguaiana) to determine their relationships and the reproductive behavior of the hybrids. One vaseygrass × Uruguayan and two vaseygrass × Uruguaiana hybrids were studied and all had 2n = 5x = 50 chromosomes. The mean pairing meiotic association for the vaseygrass × Uruguayan hybrid was 10.18 I and 19.91 II, and for the two vaseygrass × Uruguaiana hybrids was 10.28 I, 19.82 II, 0.006 III, and 0.01 IV. These findings indicate that the parents of both hybrids have two homologous genomes. Because vaseygrass and Uruguayan dallisgrass have the genome formulas IIJJ and IIJJXX, respectively, their hybrid has the formula IIJJX and the two homologous genomes are the I and J genomes. Likewise, chromosome pairing in the vaseygrass × Uruguaiana hybrids demonstrates that both species have two similar genomes, I and J. The Uruguaiana biotype usually has 8–10 univalents per cell and these result from the failure of some members of the third genome to pair. Thus the Uruguaiana biotype has different forms of the X genome, and its genome formula may be designated as IIJJXX2. Cytologically all three hybrids were facultative apomicts but their uniform F2 progeny indicates that only the apomictic embryo sacs are functional. Seed fertility ranged from 10.8 to 16.2%.Key words: Interspecific hybrids, genomes, chromosome pairing, species relationships, dallisgrass, vaseygrass.

Hybridization (N + N and 2N + N) of Facultative Apomictic Species in the Pennisetum Agamic Complex

It has been hypothesized that within agamic complexes, apomixis acts to preserve fertility in wide crosses. Through hybridization studies with two facultative (partially sexual) Pennisetum species (P. flaccidum and P. mezianum), n + n (B_II) and 2n + n (B_III) sexual, facultative, and obligate apomictic hybrids were obtained. The objectives of this study were to determine the frequency for wide-cross B_III hybrids in an agamic complex and to compare fertility of B_II and B_III hybrids. Sexual all hybrids derived from hybridization of P. flaccidum with P. mezianum were always highly sterile because of male and female abortion, while obligate apomictic B_II hybrids obtained through pseudogamy were fertile when supplied with viable pollen. All B_III hybrids from fertilization of 2n apomictic eggs were highly fertile regardless of mode of reproduction. The ability to recover obligate apomictic B_III hybrids allowed for transfer of whole genomes to the female chromosome complement and to synthesize new, fertile, true breeding 2n + n genotypes.

Diplosporous embryo-sac formation and the degree of diplospory inAllium tuberosum

To obtain information on the dynamic and quantitative aspects of displospory in a facultative apomict,Allium tuberosum (2n = 32), we cytologically observed the entire course of female meiosis and embryosac development in cv ‘Tender-Pole’ by the clearing-staining-squash method. A time-table of its course was constructed in terms of bud length and days pre-anthesis. Chromosome threads were observed in pairs at the earliest stage of meiosis, after which 32 autobivalents developed. Thus, endoreduplication must have occurred during premeiotic interphase or at the onset of female meiosis. Such endoreduplicational meiosis also occurred on the male side, though the frequency (3.9%) was much lower than that on the female side (80%). The degree of diplospory calculated as the percentage of reduplicated embryo-sac mother cells varied to some extent among six cultivars: ‘Huhehaote’ and ‘Manchuria’ showed the highest degree of diplospory (98%), and ‘Kaohsiung’ the lowest (76%).

Homology of chromosomes of the X genomes in common and Uruguayan dallisgrass, Paspalum dilatatum

Two biotypes of dallisgrass, Paspalum dilatatum Poir., designated common and Uruguayan, have chromosome numbers and genome formulas of 2n = 5x = 50 (IIJJX) and 2n = 6x = 60 (IIJJXX), respectively. The relationship between the X genomes in these two biotypes is unknown, and each was arbitrarily assigned the letter X to designate an unknown genome. This study was undertaken to determine the relationship between the X genomes in these two biotypes. Because both biotypes are apomicts and cannot be crossed, a sexual intraspecific F1 hybrid (2n = 45) between sexual yellow-anthered (2n = 4x = 40; IIJJ) and common dallisgrass biotypes was crossed with Uruguayan dallisgrass. This F1 hybrid has complete sets of the I and J genomes but only 5 of the 10 chromosomes of the X genome from common dallisgrass. Two hybrids were recovered. One had 52 and the other had 53 chromosomes, which associated at metaphase I as 22 bivalents + 8 univalents and 23 bivalents + 7 univalents, respectively. Twenty bivalents represent pairing of members of the I and J genomes, and those in excess of 20 represent pairing between members of the two X genomes. The remaining members of the X genome from the Uruguayan biotype were present as univalents at metaphase I. This demonstrates that those chromosomes of the X genome from the common biotype that were present are homologous to members of the X genome of the Uruguayan biotype. Both hybrids are aposporous facultative apomicts with some sterility.Key words: meiosis, intraspecific hybridization, chromosome pairing, genome relations, apomixis.

Cytology, Reproductive Behavior, and Forage Potential of Hexaploid Dallisgrass Biotypes

During the past 25 yr, several different dallisgrass (Paspalum dilatatum Poir.) biotypes have been discovered, but little is known regarding their cytology and reproductive behavior. This study was undertaken to determine the cytology, method of reproduction, and fertility of three biotypes and the forage yield and quality of two of them. Two biotypes, Torres and Uruguaiana, were introduced from southern Brazil and the third, Uruguayan, was collected in Uruguay. All three are hexaploids with 2n = 6x = 60. The Torres biotype was meiotically irregular, with a mean chromosome association of 58.74 univalents and 0.63 bivalents per cell. Two accessions of the Uruguaiana biotype were studied; they had a mean meiotic chromosome association of 9.62 univalents, 25.17 bivalents, and 0.01 quadrivalents. Eleven accessions of the Uruguayan biotype were studied; their mean chromosome associations were 1.26 univalents, 29.28 bivalents, and 0.04 quadrivalents. It was proposed that the lack of chromosome pairing in the Torres biotype is genetically controlled. The genomic formulas of the Uruguaiana and Uruguayan biotypes are proposed to be AABBC1C2 and AABBCC, respectively. An examination of megasporogenesis and embryo sac development revealed that all three biotypes reproduce by apomixis, with apospory followed by pseudogamy. One Uruguayan accession was a facultative apomict with a small degree of sexuality. The forage yield potential and forage quality were determined for the Torres and Uruguayan biotypes, as well as for common dallisgrass. Torres produced less forage than the other two. Common dallisgrass produced less forage than the mean of the Uruguayan accessions in most environments. In vitro dry matter disappearance (IVDMD) differences among accessions were relatively small. Common dallisgrass did not differ in IVDMD from any of the Uruguayan accessions.

Degree of apomixis in Chinese chive (Allium tuberosum) estimated by esterase isozyme analysis.

The degree of apomixis of six cultivars of Chinese chive (Allium tuberosum ROTTL.) (2n= 32, 4x) was estimated by electrophoretic analysis of leaf esterase (EST) in progenies obtained from crosses between these cultivars. In each progeny, more than 90% of the seedlings showed the same zymogram as their pistillate parent, the remaining seedlings showing hybrid zymograms. The differences between maternal and hybrid zymograms were clear and unmistakable. The degree of apomixis was therefore calculated to be higher than 90% for each cultivar tested. For two progeny arrays the results of the electrophoretic analysis were supported by simultaneous examination of two morphological traits, namely, outer tepal length and first flowering month. Among the seedlings showing the maternal EST zymogram, two dihaploids (2n= 17, 16), one each in two progenies, were detected by cytological examination of root tips. Those dihaploids being eliminated, the degree of apomixis estimated by the isozyrne analysis was reduced by 1% resp. 2% for these progenies. This modification, however, was of little significance because of the infrequent occurrence of dihaploids. The present results indicate that a progeny test exclusively based on the EST analysis gives only a slight overestimation of the degree of recurrent apomixis in Chinese chive and that Chinese chive is a facultative apomict wlth a high degree of apomixis.

Cytotaxonomic studies in Themeda triandra Forssk.: Part III: Sexual and apomictic embryo sac development in 53 collections

The embryo sac development of 53 collections of Themeda triandra Forssk. was studied. All the diploids and two of the tetraploids had sexual embryo sacs of the Polygonum -type, whereas the remaining polyploids possess 4-nucleate aposporic embryo sacs. Some of the collections are facultative apomicts, while others (in particular the penta- and hexaploids) are essentially obligate apomicts. Interesting variations in the type of apospory was encountered in the different apomicts. This variation might be due to environmental influences. On the whole it would appear from the results that Themeda triandra is a young and therefore still actively developing agamic complex in southern Africa.

EVIDENCE OF PARTIAL APOMIXIS IN ANTENNARIA MEDIA (ASTERACEAE: INULEAE) DETECTED BY THE SEGREGATION OF GENETIC MARKERS

The interplant variation in sexual and asexual reproduction in an Oregon population of the alpine perennial Antennaria media was investigated. Four polymorphic loci were assayed by enzyme electrophoresis of the progeny of 72 families from two subpopulations of A. media. The population was divided into two spatially distinct subpopulations. A multilocus model, incorporating a mixture of apomixis and random outcrossing, was used to estimate the mating system of pistillate plants both on the population and individual levels with statistical significance of the estimates based on bootstrap methods. The population contained a mixture of sexual individuals, partial apomicts, and obligate apomicts. The first subpopulation contained individuals that were partially apomictic and presumably produced both reduced and unreduced embryo sacs. There was a conspicuous difference in the breeding system composition between the two subpopulations. The first subpopulation had a “female” biased gender ratio and contained mostly obligate apomicts, some partial apomicts, and some outcrossing amphimicts. The second subpopulation, which had a nearly balanced gender ratio, contained mostly amphimicts, some obligate apomicts, but no facultative apomicts. This is the first study to document partial apomixis in individual plants by the use of genetic markers.

Evidence of Partial Apomixis in Antennaria media (Asteraceae: Inuleae) Detected by the Segregation of Genetic Markers

The interplant variation in sexual and asexual reproduction in an Oregon population of the alpine perennial Antennaria media was investigated. Four polymorphic loci were assayed by enzyme electrophoresis of the progeny of 72 families from two subpopulations of A. media. The population was divided into two spatially distinct subpopulations. A multilocus model, incorporating a mixture of apomixis and random outcrossing, was used to estimate the mating system of pistillate plants both on the population and individual levels with statistical significance of the estimates based on bootstrap methods. The population contained a mixture of sexual individuals, partial apomicts, and obligate apomicts. The first subpopulation contained individuals that were partially apomictic and presumably produced both reduced and unreduced embryo sacs. There was a conspicuous difference in the breeding system composition between the two subpopulations. The first subpopulation had a “female” biased gender ratio and contained mostly obligate apomicts, some partial apomicts, and some outcrossing amphimicts. The second subpopulation, which had a nearly balanced gender ratio, contained mostly amphimicts, some obligate apomicts, but no facultative apomicts. This is the first study to document partial apomixis in individual plants by the use of genetic markers.

A model for the evolution and control of generative apomixis

A model is presented for the evolution and control of generative apomixis—a collective term for apomixis in animals and diplosporous apomixis in flowering plants. Its development takes into account data obtained from studies of apomictic-like processes in sexual organisms and in non-apomictic parthenogens, as well as data obtained from studies of generative apomicts. This approach provides insights into the evolution and control of generative apomixis that cannot be obtained from studies of generative apomicts alone. It is argued that the control of the avoidance of meiotic reduction during egg production in generative apomicts resides at a single locus, the identity of which can vary between lineages. This variation accounts for the observed variation between taxa in the pattern of avoidance of meiotic reduction. The affected locus contains a wild-type allele that codes for meiotic reduction and excess copies of a mutant allele that codes for its avoidance. The dominance relationship between these is determined by their ratio and by the environment. Environmental differences between female generative cells and somatic cells are such that the phenotypic expression of the mutant allele is favoured in the former, while that of the wild-type allele is favoured in the latter. This is important, for the locus is also involved in the control of mitosis which would be disrupted by the expression of the mutant allele in somatic cells. The requirement to maintain a viable pattern of growth and development explains why the wild-type allele is retained by generative apomicts, and this in turn explains why the ability to produce meiotically reduced eggs is retained by facultative forms and why it appears to be suppressed in, rather than absent from, obligate forms. The requirement for excess copies of the mutant allele in generative cells explains why generative apomicts are typically polyploid, as this condition provides a simple and effective means of generating the correct balance of mutant and wild-type alleles. Environmental effects can also lead to the dominance relationship between wild-type and mutant alleles varying between generative cells. In plants, this can lead to the apomixis gene being expressed, and thus to meiotic reduction being avoided, in only some ovules. Meiotically reduced, as well as meiotically unreduced, eggs are produced when this occurs. If compatible and viable pollen is available the meiotically reduced eggs may be fertilized, resulting in these organisms reproducing as facultative apomicts. It is argued that the control and evolution of parthenogenesis in generative apomicts varies between taxa. In some, the parthenogenetic initiation of embryos may result from the acquisition of a parthenogenesis gene or genes; but there is no reason to believe that this is either a general or a common requirement. Indeed, in some it may be an ancestral trait, these apomicts differing from their sexual ancestors in the ability to mature, rather than in the ability to initiate, embryos from unfertilized eggs; or it may result from physiological or developmental changes induced, for example, by polyploidization, hybridization, or the avoidance of meiotic reduction. In some plants it may be induced by pollination (without fertilization) or by the activity of a developing endosperm. Although it is argued that most generatively apomictic lineages may have acquired this form of reproduction relatively easily, by the acquisition of a mutation at a single locus, it is argued that newly initiated lineages may often be reproductively inefficient. These will begin to accumulate mutations that improve the efficiency of apomictic reproduction. Thus several loci may be involved in the control of generative apomixis in established lineages, even though only a single locus was involved in its initiation in these lineages. Care must be taken to distinguish between these initiator and modifier genes when considering the evolution of generative apomixis. Finally, it is argued that although generatively apomictic lineages have easily acquired this form of reproduction, its evolution in some taxa may be so difficult, requiring the acquisition of mutations simultaneously at two or more loci, that these may never acquire it. Thus, evidence obtained from taxa that have successfully made the transition from sexual reproduction to generative apomixis that its evolution was straightforward should not be used as evidence that its evolution will always be relatively easily achieved. Its uneven taxonomic distribution indicates that it is much more easily evolved by some taxonomic groups than by others.

Studies in Crataegus (Rosaceae, Maloideae). XIX. Breeding behavior in Ontario Crataegus series Rotundifoliae

Ontario members of Crataegus series Rotundifoliae display breeding behavior that is characteristic of apomictic taxa. Based on a series of analyses, including histological studies of ovule development, chromosome counts, pollen stainability and pollination studies, and phenological analyses, it appears that most of the Ontario Rotundifoliae are tetraploid (2n = 2x = 68), self-fertile, pseudogamous, facultative apomicts. Some triploid (2n = 3x = 51) apomicts are also present in C. dodgei var. flavida (Sarg.) Smith & Phipps. The results presented are congruent with previous studies of Crataegus cytogenetics. Moreover, these results have assisted the taxonomic analysis of the series by permitting the identification of predominantly apomictic microspecies as well as possible hybrid taxa.

Reproduction in the New Zealand taxa ofPomaderrisLabill. (Rhamnaceae)

Abstract Evidence is presented which shows that the diploid, Pomaderris kumeraho (2n = 24), is self fertile and reproduces sexually, whereas two triploid taxa, P. hamiltonii and P. phylicifolia var. ericifolia reproduce by agamospermy. The origin of the embryo sac in the latter is diplosporous. Although breeding experiments indicate that pol~ Iination improves seed set, endosperm chromosome counts in P. hamiltonii show that fertilisation of the endosperm nucleus does not occur. Endosperm is derived from one or both polar nuclei separately. In the tetraploid P. phylicifolia var. polifolia, reproduction is mainly sexual, but there is some indication that plants may be facultative apomicts.

Degrees of sexuality in sexual plants of guineagrass by the simplified embryo sac analysis.

Degrees of sexuality in lowly and highly sexual plants of guineagratss which were isolated by progeny tests in the previous paper of the authors, were investigated by using the simplified embryo sac analysis, and compared with degrees of sexuality by frequency of off-type plants in progenies. In embryo sac analysis, ovules were classified into sexual or apomictic reproduction to detect the type of embryo sacs. Sexual embryo sac was 8-nucleate, and aposporous one was 4-nucleate. As ovules examined kept their original forms intact in pistils, reproductive behavior in polyembryonic ovules was easily identifled by determining the dominant embryo sac in multiple ones. Highly sexual diploid plants were revealed to be completely sexual based upon the embryo sac type. All ovules had single sexual embryo sacs excluding several sterile ones. Highly sexual tetraploid plant, N68/96-8, was also identified to be completely sexual, and its offsprings after open-pollination were segregated in degrees of sexuality. Five out of 10 offsprings were completely sexual, one was obligate apomictic, and the other four were facultative apomictic. Two lowly sexual plants, N68/84-1 and 73/61-9, and their offsprings after self-and open-pollination, were shown to have low frequencies of sexual embryo sacs except N68/84-1 which had rather higher rate of sexual embryo sacs. No plants with high degrees of sexuality were segregated in their offsprings. Both degrees of sexuaiity by embryo sac analysis and progeny test, agreed well excluding degrees in a few facultative apomicts. This analysis was appeared to provide more accurate estimation of degree of sexuallty. Complete sexuality observed in this study was recognized to be useful for producing controlled hybrid plants in sexual X apomictic cross in the first procedure of hybridization breeding in this apomictic grass.

Apomictic Interspecific Hybrids Between Pearl Millet and Pennisetum orientale L. C. Rich1

Pearl millet, Pennisetum americanum (L.) Leeke, is an important world food and forage crop. Pennisetum orientale L. C. Rich has genes for apomixis, perennial growth habit, pest resistance, and drough tolerance which could be used to improve pearl millet. The objectives of this research Were to determine the cytotaxonomic relationship of these two species and to explore the feasibility of interspecific germplasm transfer. Five interspecific hybrids, 2n = 25, with 7 large P. americanum millet (A) and 18 small P. orientale (0) chromosomes were produced by pollinating cytoplasmic male sterile pearl millet with P. orientale pollen. The O chromosomes paired mainly as bivalents and the A chromosomes remained as univalents. A low frequency of AO chromosome associations were observed. Although the possibility of germplasm exchange existed, the two species appeared to be not closely related. Among three hybrids examined, one was a facultative apomict, one was an obligate apomict and another was highly apomictic with 3% of ovules with sexual embryo sacs. Sixteen backcross progenies were established from interspecific hybrids pollinated with pearl millet pollen. Seven plants were 2n = 23 with 14 A + 9 0 chromosomes, five were 2n = 27 with 7 A + 20 O chromosomes and four were 2n = 32 with 14 A and 18 O chromosomes. The balanced chromosome number for both species in these latter plants should provide a mechanism for restoring fertility in the interspecific hybrid thus enabling germplasm transfer. The interspecific hybrids were male sterile but set about 1% seed when pollinated with pearl millet pollen.

Evolution of Reproductive Systems in the Gramineae

From a simple hermaphrodite flower, and from a complex incompatibility system unique among the flowering plants, several breeding systems have evolved in the Gramineae. Self-compatibility is the most commonplace variant and following this mutation, habitual or facultative cleistogamy is a simple evolutionary step. Separation of the sexes to different plants as in dioecism is relatively uncommon (ca. 20 genera), and gynodioecism is much less frequent still (3-4 genera). Both are seen as escape pathways from self-compatibility; the development of such pathways is discussed. Separation of the sexes to separate flowers as in monoecism is relatively common and with the variants andromonoecism and gynomonoecism is the most substantial departure from hermaphroditism in the family. These states are also interpreted as responses to self-compatibility; and though they do not generate cross-fertilization, they assist its evolution. Pathways for the evolution of these breeding systems are described. Apomixis and the breeding system best suited are discussed.I Reproductive biology in the Gramineae begins at the transition in the shoot apex from leaf production to the initiation of inflorescence primordia and the later development of floral structures. These have been well described for numerous grasses (Barnard, 1955, 1957, 1964; Bonnett, 1966; Sharman, 1960), and are mediated by photoperiod. The review of Evans (1964) elegantly reveals data on the interplay of daylength, temperature, and vernalization on inflorescence development. Floral induction and initiation may occur in the season of flowering (Evans, 1964), or in the season preceding inflorescence emergence (Mark, 1965; Hodgson, 1966). Inflorescence emergence is temperature or daylength dependent (Cooper, 1952; Connor, 1963; Heslop-Harrison, 1961). Temperature also controls anthesis, and later the release of pollen from anthers. Photoperiod, however, has other effects on the reproductive cycle. It may, for example, affect the frequency of cleistogamy in facultatively cleistogamous grasses (Langer & Wilson, 1965), or in a facultative apomict the frequency of apomictic or sexual embryo sacs (Knox, 1967; Knox & Heslop-Harrison, 1966), or depress maleness (Heslop-Harrison, 1959), or reduce the number of florets in the male inflorescence of Zea mays (Moss & Heslop-Harrison, 1968), or control protandry and protogyny (Emerson, 1924). Although these environmental influences are considerable, genetic influences may cause transient male and female sterility, or restore lost fertility in part or in whole, or promote redistribution of the sex-forms. A combination of both genetic and physiological factors may introduce difficulties into an interpretation of the breeding system of any grass. At the International Symposium on Reproduction in Flowering Plants held at Christchurch, New Zealand, in 1979, I presented a survey of the breeding systems known in the Gramineae (Connor, 1980). This present paper is complementary 1 J am grateful to colleagues and friends in several countries for helpful discussion, and especially to D. W. Clayton, Kew, T. R. Soderstrom, Smithsonian Institution, Ana M. Anton, Universidad Nacional de C6rdoba, Elizabeth Edgar and C. J. Webb, Botany Division, DSIR. 2 Botany Division, Department of Scientific and Industrial Research, Christchurch, New Zealand. ANN. MISSOURI BOT. GARD. 68: 48-74. 1981. 0026-6493/81/0048-0074/,$2.85/0 This content downloaded from 157.55.39.132 on Thu, 15 Sep 2016 05:44:04 UTC All use subject to http://about.jstor.org/terms 1981] CONNOR-REPRODUCTIVE SYSTEMS IN THE GRAMINEAE 49 to that; the bibliographies of both papers supplement each other. Here, as there, I follow the organization of tribes proposed by Hubbard (1973) for the grasses. THE FLOWER, THE SPIKELET, AND THE INFLORESCENCE The primitive grass flower is postulated as having three bracts and lodicules, six stamens, and a 1or 3-locular tristigmatic gynoecium (Schuster, 1910; Arber, 1934; Clifford, 1961). The spikelet is considered to have been many flowered, and the inflorescence is thought to have comprised many of these spikelets in a simple moreor less-branched, terminal panicle. Associated with these characteristics is the further postulate of entomophily (Schuster, 1910; Clifford, 1961; Stebbins,

Facultative Apomixis in Eragrostis curvula1

Previous reports indicate that weeping lovegrass, [Eragrostis curvula (Schrad.) Nees], reproduces either sexually or by obligate apomixis (diplospory). Recent studies have shown that certain plants of this species are facultative apomicts. Our research objective was to determine the cytological basis for facultative apomixis in weeping lovegrass and to relate reproductive cytology to performance of progeny from lovegrass parents and hybrids. We observed enlarged or late megaspore mother cells (MMC), as determined by integument size, in apomictic strains. Development from these MMCs occurred in either of two ways: 1) direct formation of the embryo sac (diplospory), or 2) delayed meiosis resulting in the formation of a linear tetrad. Progeny tests of plants with both types of reproductive cytology suggests that these patterns indicate facultative apomixis. We did not observe late, greatly enlarged megaspore mother cells or delayed meiosis in sexual plants. Hybrids between sexual plants and highly apomictic common weeping lovegrass were observed to reproduce either sexually or by facultative apomixis. Sexual plants, when self‐pollinated, produced no apomictic offspring. Therefore, we concluded that the sexual plants studied probably lack all the necessary genes for expression of apomixis. The uniformity observed in most experimental lines, the low level of sexuality in some apomictic hybrids, and the reduced vigor of many of the aberrant plants suggest that varietal development and maintenance in E. curvula should not be hindered too greatly by facultative apomixis.

Stubby Head, an Induced Facultative Apomict in Pearl Millet1

Stubby head is a multi‐ovary, ‐ovule, ‐embryo sac mutant induced in pearl millet (Pennisetum typhoides (Burm.) Stapf. and Hubb.) by a dual treatment of the thermal neutrons and diethyl sulfate. The mutant has shortened internodes and flattened stems and produces a rosette of leaves around a stubby inflorescence. Cytological tests and progeny tests indicate that it is a facultative apomict. This mutant could make a significant contribution to the improvement of pearl millet if it can be developed into a usable form.

Chromosome and Genetic Constancy in a Poa pratensis L. Cultivar1

Chromosome numbers were determined on genetically identifiable plants to evaluate chromosomal and genetic constancy of the ‘Windsor’ cultivar of the facultative apomict Poa pratensis L. Seed size, seedling pigmentation, and chromosome number were interrelated in most plants but varied within the cultivar.Large seeds produced 93% pigmented seedlings, of which 84% had 70 chromosomes. Small seeds produced 86% nonpigmented seedlings, of which 94% had 49 chromosomes. Although the cultivar consisted largely of these two relatively stable components, chromosome numbers varied among plants with inverse associations of seed size and seedling pigmentation. Throughout the experiment the following chromosome numbers were found: 35, 49, 56, 60's, 70's, 80's, and 105. A set of twin plants occurred, with one member with 49 chromosomes and the other with 74 chromosomes. The wide range of chromosome numbers was accounted for on the basis of classical explanations of agamospermy in facultative apomicts.

Aneuploidy and Apomixis in Bothriochloa and Dichanthium

1. Hybrids were produced among facultative apomictic biotypes within Bothriochloa intermedia, B. grahamii, and Dichanthium annulatum. 2. Assumed natural hybrids were euploid and facultative apomicts. 3. Artificially produced F1 and F2 hybrids were either euploid or aneuploid, and some reproduced strictly by sexual means. 4. Sexually reproducing plants usually gave rise to F2 progenies characterized by a high degree of aneuploidy. 5. The absence of truly sexual races in nature was explained on the basis that the over-all apomictic mode of reproduction is inherited as a dominant character over sexuality. 6. The absence of aneuploidy among natural populations suggested that euploids must have a selective advantage over aneuploids.

Chromosome Number and Mode of Reproduction in theBothriochloininae

SUMMARYThe genus Bothriochloa is characterized by chromosome numbers ranging almost continuously in multiples of 10 form 2n = 20 to 2n=180. Chromosome numbers of 2 n = 20, 40 and 60 were encountered in Dichanthium. The genus Capillipedium is characterized by 2n=20, 40 and 50 chromosomes, while representatives of the genera Euclasta and Eremopogon, which were studied, are tetraploids (2n = 40). In these genera diploids (2n = 20) reproduce sexually, tetraploids are mostly facultative apomicts, and higher polyploids are essentially obligate apomicts. Exceptions are the American and some Australian polyploid representatives of Bothriochloa (2n = 60, 80, 120, 180) which reproduce sexually. The diploids are all narrow endemics, the facultative apomicts are widely distributed, and the sexually reproducing as well as apomictic polyploids are usually restricted in their distribution range.