Phytogeographical studies ofCalamagrostis sachalinensis (Gramineae)

Chromosome numbers were determined on 223 collections ofCalamagrostis sachalinensis from 18 localities in Japan. The plants were found to be tetraploid (2n=28), hexaploid (2n=42) or octoploid (2n=56). A few collections were found to include one or two B-chromosomes. The tetraploid collections were made from central Honshu and Mt. Apoi in Hokkaido, while the hexaploids and the octoploids were detected in many localities. Pollen examination of these collections showed that the tetraploids with but one exception have good pollen and the hexaploids and the octoploids have no pollen or have bad pollen with stainability less than 10%. With the help of pollen examination of a number of herbarium specimens, the distribution of the tetraploids and that of the assemblage of the hexaploids and octoploids were delineated. Morphological studies indicated that the tetraploid, hexaploid and octoploid plants can not be separated in gross and spikelet morphology and that the tetraploids in central Honshu and those in Mt. Apoi are significantly different in leaf features. It was concluded thatC. sachalinensis represents an apo-amphimictic complex, which includes the following four races: 1) tetraploid, amphimictic, having thin leaf blades 5–10 mm broad and growing on the subalpine conifer forest belt and the conifer forest-alpine ecotone in the mountains of central Honshu; 2) tetraploid, amphimictic, having hard leaf blades 2–6 mm broad and growing on the stony, arid and exposed alpine belt on Mt. Apoi in Hokkaido; 3) hexaploid, mainly apomictic, the most variable ecologically, widely distributed; 4) octoploid, mainly apomictic, frequent in the upper montane to alpine belts, probably widely distributed.     

NATURAL AND ARTIFICIAL HYBRIDS OF HELIANTHUS MAXIMILIANI X H. GROSSESERRATUS

Long , Robert W. (Ohio Wesleyan U., Delaware.) Natural and artificial hybrids of Helianthus Maximiliani × H. grosseserratus. Amer. Jour. Bot. 46(10): 687–692. Illus. 1959.—An investigation of the occurrence of natural hybridization in two perennial sunflowers, Helianthus Maximiliani and H. grosseserratus, was begun in 1950. Subsequently, artificial F₁, F₂, and first and second backcross generations were produced. Fertility and vigor were high in all these plants, but F₁ plants appeared to excel the others in these characteristics. Observations in the experimental garden were supplemented by examination of chromosomes in pollen mother cells, comparisons of herbarium collections, and study of wild populations. Evidence pointed to close genetic relationship of the species and to the occurrence of natural hybridization in areas of distributional overlap. In 1957 and 1958, field work in these areas resulted in the scoring of 18 natural populations, 3 of which consisted of both parental species plus putative F₁ hybrids. Two explanations are offered to account for the seeming absence of introgression. The results support the conclusion that natural hybridization leads to the establishment of F₁ hybrids and that introgression does not occur to any significant extent. Although both species display a high degree of interfertility, they are distinct morphologically. For this reason, it is advisable to maintain them as separate species.     

Transmission of chromosomes through the eggs and pollen of triticale × wheat F1 hybrids

Summary The substitution patterns of rye chromosomes in hexaploid triticale × wheat F₂ hybrids, along with the transmission patterns of rye chromosomes through egg cells and pollen when several of the F₁ hybrids were test crossed to triticale and wheat were investigated. The data indicated that the rye chromosome transmission through both the egg and pollen was random in number and in composition. The test crosses suggested that it was best to use wheat pollen for the transmission of rye chromosomes through the egg cells of the F₁ hybrids and triticale egg cells for the transmission of rye chromosomes through F₁ hybrid pollen. A deviation from random segregation in the F₂ and the transmission rate was observed for rye chromosomes 1R, 4R/7R, and 6R. The transmission rates of 1R and 6R varied depending on the direction in which the cross was made. The results also indicated that there was little or no compensation between the R- and D-genomes and that the chromosomes of these two genomes appeared to be transmitted independently of each other.     

The influence of hybridization on epidermal properties of birch species and the consequences for palaeoclimatic interpretations

The Fennoscandian birch population primarily consists of Betula nana, B. pendula and B. pubescens ssp. czerepanovii, the Mountain birch. Frequent hybridization between the Mountain birch and B. nana generates a wide range of genotypic and phenotypic plasticity in the subarctic birch zone of Fennoscandia. Phases of subarctic conditions prevailed during the Late Glacial in large parts of NW Europe, and palynological as well as macrofossil analysis provide some evidence for the occurrence of birch hybrids during these intervals. Leaves from genetically controlled specimens of Betula pendula, B. pubescens ssp. czerepanovii, B. nana and the hybrids B. pubescens ssp. czerepanovii × nana and B. nana × pubescens ssp. czerepanovii are investigated for their specific characteristics of the epidermis morphology. Frequency and size of epidermal cells and stomata reveal a close affinity of both hybrids to B. nana and allow a differentiation of the intermediate forms between B. nana and the Mountain birch. With respect to palaeoatmospheric CO₂ reconstructions based on stomatal index, epidermal analysis shows that a possible occurrence of hybrids in fossil leaf assemblages has no profound consequences for combined species records. However, the significant differences observed in B. nana demand the separation of this species. A comparison of the cuticle properties of B. pendula and B. pubescens from Finnish Lapland and leaf material from The Netherlands reveals a divergence of the stomatal index that may be due to differences in day light length.     

Characteristics of pollen from natural triploid Betula hybrids

Birch has a key role in the Holocene vegetation history of northern Europe and in sub‐arctic climates dwarf birch and tree birch co‐exist. In Iceland, triploid hybrids between diploid Betula nana (dwarf birch) and tetraploid B. pubescens (downy birch) are common and therefore likely to contribute to pollen deposition. Pollen from 22 triploid trees/shrubs from ten woodlands in Iceland was examined and its size and shape compared with pollen from the parental species. The mean diameter of pollen grains from the triploid hybrids was not statistically different from that of B. nana pollen, but was significantly smaller than the mean value of B. pubescens pollen. On the contrary, the size of the vestibulum was similar to that of B. pubescens, which was significantly greater than that of B. nana, and therefore the diameter‐pore depth ratio was lower than the values from either species. The pattern of size distribution within plants indicated that triploid hybrids might have produced two sizes of triporate pollen grains, but the small B. nana size was far more prevalent than the larger B. pubescens size. Several anomalies in pollen morphology were common among the hybrid pollen grains: four or more pores were the most frequent type of abnormality. Characteristics of the pollen of triploid Betula hybrids, especially structural anomalies, may provide a means to reveal periods of interspecific hybridisation in the analysis of sub‐fossil pollen.     

Durum wheat as a candidate for the unknown female progenitor of bread wheat: an empirical study with a highly fertile F<Subscript>1</Subscript> hybrid with <Emphasis Type="Italic">Aegilops tauschii</Emphasis> Coss.

Hexaploid bread wheat was derived from a hybrid cross between a cultivated form of tetraploid Triticum wheat (female progenitor) and a wild diploid species, Aegilops tauschii Coss. (male progenitor). This cross produced a fertile triploid F₁ hybrid that set hexaploid seeds. The identity of the female progenitor is unknown, but various cultivated tetraploid Triticum wheats exist today. Genetic and archaeological evidence suggests that durum wheat (T. turgidum ssp. durum) may be the female progenitor. In previous studies, however, F₁ hybrids of durum wheat crossed with Ae. tauschii consistently had low levels of fertility. To establish an empirical basis for the theory of durum wheat being the female progenitor of bread wheat, we crossed a durum wheat cultivar that carries a gene for meiotic restitution with a line of Ae. tauschii. F₁ hybrids were produced without using embryo rescue techniques. These triploid F₁ hybrids were highly fertile and spontaneously set hexaploid F₂ seeds at the average selfed seedset rate of 51.5%. To the best of our knowledge, this is the first example of the production of highly fertile F₁ hybrids between durum wheat and Ae. tauschii. The F₁ and F₂ hybrids are both similar morphologically to bread wheat and have vigorous growth habits. Cytological analyses of F₁ male gametogenesis showed that meiotic restitution is responsible for the high fertility of the triploid F₁ hybrids. The implications of these findings for the origin of bread wheat are discussed.     

Taxonomic studies of the genusPoa of Japan

With the aim of making clear the boundaries between species in thePoa acroleucahisauchii-nipponica aggregate, chromosomes and morphological features of 746 collections gathered from 125 localities in Japan were examined. For morphological observations, the voucher specimens of 95 collections whose chromosome numbers were reported previously (Tateoka, 1985) were also used. Tetraploids (2n=28) and hexaploids (2n=42), as well as a few pentaploids which were the hybrids between 4× and 6×, were found. By examining morphological features of these collections, two groups were recognized in tetraploids and one in hexaploids. The two tetraploid groups corresponded toPoa acroleuca Steud. andP. hisauchii Honda, and the hexaploid group toP. nipponica Koidz. It was confirmed that the hairiness on the internerve surface of lemma, ligule hairiness and the length ratio of anther/ lemma are the most important features for discriminating between these species; panicle shape, leaf shape and anther length are also helpful for the identification. The ambiguity of the boundaries between species which was hitherto present in the taxonomy of this species aggregate was unrelated to the creation of nature itself but was attributable to the insufficiency of our research work.     

Pollen exine ornamentation in the F2 generation of an interspecific hybrid of Chorisia (Bombacoideae,Malvaceae) in relation to inheritance pattern

Pollen morphology in ten plants of F₂ progeny of an interspecific tree hybrid, Chorisia insignis H. B.& K.×C. speciosa St. Hil (Bombacoideae, Malvaceae) has been studied with a view to have an insight in to the effect of hybridization on the pollen morphological features in F₂ generation. The study is a sequel to an earlier study on the pollen morphology of Chorisia species and their F₁ hybrid, in which case the hybrid pollen uniformly exhibited the apertural features of the male parent and exine features of the female parent. In the F₂ progeny the pollen grains display the apertural feature of the male parent in all the plants. However, with regard to exine ornamentation, variability has been observed. Of the ten plants, two plants exhibit the exine features of the male parent showing empty lumina (without bacula), five plants have their pollen exactly like that of the F₁ plant (showing prominent columellar heads in the lumina of the apocolpium region) and the remaining three plants showing columellae in lumina of both apocolpium and mesocolpium region being different from the above types. It has been inferred that the variability in the exine pattern in F₂ pollen indicates that the pollen exine pattern in hybrids, perhaps, is not unequivocally controlled by sporophytic or gametophytic genomes.     

Development of a fertile genetic bridge between Trifolium ambiguum M. Bieb. and T. repens L.

  Trifolium ambiguum M. Bieb and T. repens L. are taxonomically related but very difficult to cross. The rare hybrids so far reported between these two species were obtained only by embryo culture. This difficulty has been overcome in the present research by the creation of a “fertile bridge” between T. ambiguum and T. repens. Characters of interest can now be transferred from T. ambiguum to T. repens by using this “fertile bridge” without the use of sophisticated techniques. An array of backcross progenies was generated from crosses between a T. ambiguum×T. repens F₁ hybrid (8x H-435) and its parental species. The 8x hybrid was cross-fertile only with T. repens and resulted in 145 seeds from 1578 reciprocal crosses. Eleven of nineteen initially grown BC₁F₁ plants were all hexaploid with an average pollen stainability of 41.6%. A high frequency of multivalents at metaphase-I indicated that both autosyndetic and allosyndetic pairing occurred. Backcrosses of 6x BC₁F₁ plants to T. repens resulted in 5x BC₂F₁ plants with an average pollen stainability of 59.3%. On the other hand, 6x BC₁F₁×6x T. ambiguum crosses did not produce any seed and only two pentaploid plants were obtained from 6x BC₁F₁×4x T. ambiguum crosses. The difficulty encountered in generating 6x backcross progeny with 6x T. ambiguum was overcome by intercrossing the 6x BC₁F₁ plants and producing 6x BC₁F₂ plants with an average pollen stainability of 65.8%. One of these 6x BC₁F₂ plants was cross-compatible as a female with 6x T. ambiguum and resulted in CBC₂ plants that were all cross-compatible with 6x T. ambiguum. The 6x BC₁F₂ plants are likely to be superior to 6x BC₁F₁ progeny, as they have exhibited better expression of the combined rhizomatous and stoloniferous growth habit, improved fertility, more frequent nodal rooting and heavier nodulation. Consequently, the 6x BC₁F₂ plants can either be used directly in the selection programme or as a “fertile bridge” between the two parental species. The present work has resulted in the development of a series of fertile hybrids by the manipulation of chromosome numbers, combining the agronomic characteristics of the parent species in varying genome balances and at a range of ploidy levels. It is concluded that the initial sterility of the primary interspecific hybrids need not be a barrier to successful inter-breeding. Received: 2 August 1996 / Accepted: 4 April 1997     

Materials for the rust flora of Japan VI

Ten rust species are reported with new information on range of distribution and host relations. Among these,Puccinia fagopyricola andUromyces junci were new to Japan, andAecidium araliae was newly found in northern Honshu. Additional collections were made ofBlastospora smilacis, Puccinia malvacearum andP. orbicula in northern Honshu. New hosts were added toColeosporium solidaginis, C. tussilaginis, Phragmidium miyakeanum andPuccinia caricis, and a new Japanese host toPuccinia malvacearum.V: Trans. Mycol. Soc. Japan29: 471–478, 1988.     

Specifity of Genetic Determination of Content of Photosynthetic Pigments in Triticale

Lines of winter hexaploid Triticale and their F₁ and F₂ hybrids differing in morphological structure, pigment contents, photosynthetic productivity, and grain crops were studied. F₁ hybrids received by crossing of Triticale lines contrasting in pigment contents showed in some cases a heterosis effect for chlorophyll (Chl) content per unit leaf area. Variation analysis demonstrated a polygenic control of Triticale pigment contents, and different rate of increase in F₂ generation. We found maternal type of heritability of Chl b content and Chl content in light-harvesting complex of photosystem 2.     

Natural hybridization in JapaneseCalamagrostis

Relationships amongCalamagrostis longiseta var.longiseta, C.l. var.longe-aristata andC. fauriei, which are principally at the 2n=28 chromosome level, were studied mainly on the basis of population samples. The populations comprised (1) “pure” populations of each ofC.l. longiseta, C.l. longe-aristata andC. fauriei and (2) populations showing a gradation fromC.l. longiseta toC. fauriei via the intermediateC.l. longearistata and fromC.l. longiseta toC.l. longe-aristata. Hybridization betweenC.l. longiseta andC. fauriei was estimated to be ultimately related to the geneses of the population structures detected, and it was concluded thatC.l. longe-aristata must be of hybrid origin betweenC.l. longiseta andC. fauriei. Populations showing a gradation fromC.l. longiseta toC.l. longe-aristata were regarded to have been brought about by the extinction ofC. fauriei from the hybridizing population coupled with the change of environments and also by the secondary contact betweenC.l. longiseta and plants ofC.l. longe-aristata migrating from the area that produced it. No significant reduction of pollen fertility and seed productivity was found inC.l. longe-aristata. Based on the results of population analyses, the delimitation of the taxa concerned was made clearer. The examination of herbarium specimens showed that the range ofC.l. longe-aristata considerably exceeds the overlapping area of the present ranges ofC.l. longiseta andC. fauriei. The three taxa were deviated ecologically, and “pure” populations ofC.l. longe-aristata were found on several damp subalpine fields in central and northern Honshu that seemed to represent the most suitable niche for this taxon. It was estimated thatC.l. longe-aristata would evolve into a new good species if the future organism-environment interactions are favorable for it.     

Microsatellite mapping of a Triticum urartu Tum. derived powdery mildew resistance gene transferred to common wheat (Triticum aestivum L.)

A powdery mildew resistance gene from Triticum urartu Tum. accession UR206 was successfully transferred into hexaploid wheat (Triticum aestivum L.) through crossing and backcrossing. The F₁ plants, which had 28 chromosomes and an average of 5.32 bivalents and 17.36 univalents in meiotic pollen mother cells (PMC), were obtained through embryos rescued owing to shriveling of endosperm in hybrid seed of cross Chinese Spring (CS) × UR206. Hybrid seeds were produced through backcrossing F₁ with common wheat parents. The derivative lines had normal chromosome numbers and powdery mildew resistance similar to the donor UR206, indicating that the powdery mildew resistance gene originating from T. urartu accession UR206 was successfully transferred and expressed in a hexaploid wheat background. Genetic analysis indicated that a single dominant gene controlled the powdery mildew resistance at the seedling stage. To map and tag the powdery mildew resistance gene, 143 F₂ individuals derived from a cross UR206 × UR203 were used to construct a linkage map. The resistant gene was mapped on the chromosome 7AL based on the mapped microsatellite makers. The map spanned 52.1 cM and the order of these microsatellite loci agreed well with the established microsatellite map of chromosome arm 7AL. The resistance gene was flanked by the microsatellite loci Xwmc273 and Xpsp3003, with the genetic distances of 2.2 cM and 3.8 cM, respectively. On the basis of the origin and chromosomal location of the gene, it was temporarily designated PmU.     

RELATIONSHIP OF WOODY PARTHENIUM ARGENTATUM AND HERBACEOUS P. HISPIDUM VAR. AURICULATUM (ASTERACEAE)

Interspecific hybrids between woody Parthenium argentatum Gray (guayule), native to Mexico and Southwest Texas, and herbaceous perennial P. hispidum var. auriculatum (Britton) Rollins, native to the United States, were obtained successfully. The F₁ hybrids were intermediate for most morphological characters with the exception of the short woody stem, yellow pollen color, and the trichome morphology. Chromosome counts revealed the presence of 2n = 36 A-chromosomes in P. argentatum. The same number of A-chromosomes and four B-chromosomes were found in P. hispidum var. auriculatum. Observations of pollen mother cells showed regular meiosis in both parental species. At diakinesis, chiasmata averaged 1.12 and 1.24 per bivalent for P. argentatum and P. hispidum var. auriculatum, respectively. Meiotic behavior of the F₁ hybrids was irregular. F₁ hybrids averaged 4.43 univalents at metaphase I, 1.95 laggards at anaphase I, and 1.62 micronuclei at the tetrad stage. The low pollen stainability (5.1%) in the F₁ hybrids and the limited number of viable BC₁ seeds (4.07%) may be reflections of the irregular meiosis. Although these primary hybrids are partially fertile, they can be used to introduce desirable characteristics of P. hispidum var. auriculatum, such as herbaceous perennial habit, regrowth ability, and cold tolerance into guayule.     

Agrostis mertensii (Poaceae) in hokkaido

Populations of theAgrostis flaccida-mertensii complex in Hokkaido were examined as part of the taxonomic study of this complex, which has a wide range and comprises a polyploid series of 2x–8x (x=7). The principal aim was to clarify the taxonomic status of hexaploids growing in Hokkaido. Field explorations were made in the mountains and on the coasts in Hokkaido, and the materials were gathered from 19 localities. The occurrence of four cytotypes (2x, 4x, 6x, 8x), as well as some triploids and septaploids, of this complex in Hokkaido was confirmed, and the ranges of respective cytotypes were delineated. Morphological examinations showed that diploids, triploids and tetraploids are referable toAgrostis flaccida and octoploids toA. mertensii. The hexaploids of Hokkaido were morphologically different fromA. flaccida and also from the related hexaploid species,A. tateyamensis, distributed in Honshu. The hexaploids were quite similar to octoploidA. mertensii in all the morphological features examined and also in habitat preference. Septaploids were sterile hybrids between hexaploids and octoploids. A conclusion was drawn that the hexaploid plant of Hokkaido should be grouped with the octoploid in one and the same species,Agrostis mertensii Trin.Agrostis mertensii in this amended sense is discussed from a cytogeographical viewpoint.     

Use of <Emphasis Type="Italic">2n</Emphasis> gametes for the production of sexual polyploids from sterile Oriental × Asiatic hybrids of lilies (<Emphasis Type="Italic">Lilium</Emphasis>)

Sixteen Oriental and 12 Asiatic cultivars were crossed in 158 different combinations. A total of 708 F₁ hybrids were obtained from 86 of the different combinations of 15 Oriental and 11 Asiatic cultivars. Because the Lilium cultivars (2n=2x=24) used for the production of these hybrids belong to two different taxonomic sections—Archelirion (O) and Sinomartagon (A), respectively—the F₁ hybrids (OA) could be obtained only through embryo, embryo sac rescue, ovary slice or ovule culture. Most of the F₁ hybrids were highly sterile (did not produce viable n gametes) due to the failure of chromosome pairing. However, in a few cases F₁ plants were found that produced viable 2n pollen at variable frequencies. These 2n pollen grains were successfully used for the production of backcross progenies. Using genomic in situ hybridization we found intergenomic recombinant chromosomes in the sexual polyploid progenies. These results indicate that there are effective prospects for combining important horticultural traits from the two main groups of cultivars of lilies through sexual polyploidization.     

Doubled haploids of novel trigenomic Brassica derived from various interspecific crosses

To develop doubled haploid (DH) mapping populations of hexaploid Brassica, 10 F₁ hybrids derived from crosses between allohexaploid Brassica parents were evaluated in this study. The allohexaploid Brassica parents were selfed progenies of unique interspecific crosses between Brassica rapa (genome AA) × B. carinata (BBCC), B. nigra (BB) × B. napus (AACC), and a complex cross between B. juncea (AABB), B. napus and B. carinata, with relatively stable chromosome number (2n = 54). Hexaploid status and chromosome behavior during meiosis I in four promising F₁ hybrids were assessed using microscopy and flow cytometry, and progeny were obtained following microspore culture. Hybrids H11-2 and H16-1 demonstrated higher amenability for embryo generation, plantlet regeneration, and frequency of production of DH microspore-derived progeny of hexaploid DNA content (6x) compared to hybrids H08-1 and H24-1. A total of 370 6x DH progeny were selected out of 693 plantlets from H11-2, 241/436 from H16-1, 23/54 from H08-1, and 21/56 from H24-1. DH progenies of hybrids H11-2 and H16-1 were then designated as promising mapping populations of a new hexaploid Brassica species.     

Attributes ofCalamagrostis langsdorffii,C. sachalinensis and their intermediates in Shikoku

The internal structure of theCalamagrostis langsdorffii-sachalinensis complex in Shikoku was examined on the basis of a number of new collections. Chromosome counts disclosed the occurrence of the following entities: 6X and 8X, as well as some aneuploids, ofC. longsdorffii; 6X ofC. sachalinensis; 6X and 8X of the intermediates between these two species. The examination of pollen of these collections and the herbarium specimens showed that all of them may be apomictic in seed formation and that the sexual tetraploids of the pertinent complex may not be distributed in Shikoku. Details of morphological features of the collections were analyzed, and the intermediacy in morphology of the group “intermediate” was confirmed by the distance diagram method. The collections ofC. langsdorffii from Shikoku generally showed an awn longer than that of the collections from Honshu and Hokkaido. On the basis of these and some other observations, relationships of the strains extant in Shikoku were discussed. For the derivation of the present-day composition of the pertinent complex, an hypothesis which is founded on the presumption of a past occurrence of sexual tetraploids of bothC. langsdorffii andC. sachalinensis in Shikoku has been advanced.     

Reproductive interactions between hybridizing irises: analyses of pollen-tube growth and fertilization success

Pollen-tube growth and seed siring ability were measured in crosses between the Louisiana iris species Iris fulva and Iris hexagona and their F₁ and F₂ hybrids. Flowers of the parental species were pollinated with self, outcross intraspecific, and interspecific pollen. Pollen-tube lengths were similar for all three pollen types in I. fulva, but in I. hexagona interspecific pollen tubes were longer than intraspecific pollen tubes. Pollen-tube lengths also differed for F₁ and F₂ flowers pollinated with I. fulva, I. hexagona, and hybrid pollen. For both hybrid classes I. fulva pollen tubes were the shortest while pollen tubes from I. hexagona and hybrids grew the furthest. Mixtures of genetically marked pollen were used to assess the seed siring ability of intra- and interspecific pollen in the parental species by varying the proportion of each pollen type in a replacement series design. For both species, the observed proportions of hybrid seeds were lower than the expected based on the frequency of each pollen type in the mixtures across all treatments. Flowers of I. fulva produced less than 10% hybrid progeny even when 75% of the pollen applied to stigmas was derived from interspecific flowers. The frequency of hybrid seed formation was somewhat greater in I. hexagona, but was still significantly lower than expected across all mixture treatments. Seed set per fruit remained constant across the mixture treatments for both species, but in I. fulva fruit set decreased with an increase in the proportion of interspecific pollen. The data indicate that both pre- and postfertilization processes contribute to discrimination against hybrid seed formation.     

SEMISTERILITY IN THE INTERSPECIFIC HYBRID MELILOTUS POLONICA × M. ALBA

Jaranowski , J. K. (Coll. of Agriculture, ul. Wojska Polskiego 71c, Poznan, Poland.) Semisterility in the interspecific hybrid Melilotus polonica × M. alba. Amer. Jour. Bot. 48(1): 28–35. Illus. 1961.—Interspecific hybrids between Melilotus polonica (n = 8) and M. alba (n = 8) are readily secured. The F₁ hybrids are intermediate between the parents and partially sterile with a mean percentage of 58.8 (ranging from 46.8 to 72.6) defective pollen grains. Six bivalents and a chain or ring of 4 chromosomes occur at diakinesis and metaphase-I of microsporogenesis. A crossshaped configuration characteristic of a reciprocal translocation is present at pachytene, indicating that one of the parents is homozygous for an interchange of relatively large section between two of the members of the chromosome complex. Chromosome bridges, lagging chromosomes, movement of the univalents to the same pole and precocious division of the univalents lead to aberrant chromosome distribution during the course of meiosis. Reduction in self-fertility indicates a corresponding aberrant distribution of chromosomes during megasporogenesis. Pollen sterility in the F₂ generation ranged from 24.8% to 72.5% with a mean value of 54.6%. Two plants in the F₂ generation which had relatively low pollen sterility proved to be aneuploids (2n + 1). Meiotic irregularities in the F₂ plants were comparable to those exhibited by the F₁ plants.