Frequency Of Trivalents Research Articles

A comparative cytogenetic study of the rice (Oryza sativa L.) autotetraploid restorers and hybrids

We studied pollen fertility, seed set and cytogenetic characteristics of restorer lines and F1 hybrids of autotetraploid rice. T4002, T4063, T461A x T4002 and T461A x T4063 showed significantly higher pollen fertility and seed set than T4132 and T461A x T4132. Meiotic pairing configurations of T4002, T4063, T4132, T461A x T4002, T461A x T4063 and T461A x T4132 were 0.051 + 19.96II (9.89 rod + 10.07 ring) + 0.01III + + 2.00IV, 0.11I + 19.17II (8.90 rod + 10.37 ring) + 0.09III + 2.26IV + 0.01VI, 1.34I + 9.46II (4.50 rod + 4.96 ring) + 0.80III + 6.02IV + 0.09VI + 0.09VIII, 0.02I + 14.36II (6.44 rod + 7.91 ring) + 0.01III + 4.80IV + 0.01VIII, 0.06I + 17.67II (11.01 rod + 6.67 ring) + 0.06III + 3.10IV + 0.01VI and 1.11I + 11.31II (5.80 rod + 5.51 ring) + 0.41III + 5.63IV + 0.03VI + 0.03VIII, respectively. Configuration 16II + 4IV and 12II + 6IV occurred in the highest frequency among the autotetraploid restorers and hybrids. Meiotic chromosome behaviors were less abnormal in the tetraploids with high seed set than those with low seed set. The hybrids had fewer frequencies of bivalents, univalents, trivalents and multivalents than the restorers, but higher frequency of quatrivalents than the restorers at MI. The frequency of univalents at MI had the most impact on pollen fertility and seed set, i.e., pollen fertility decreased with the increase of univalents. The secondary impact factors were trivalents and multivalents, and bivalents and quatrivalents had no effect on pollen fertility and seed set. The correlative relationship between pollen fertility and cytogenetic behaviors could be utilized to improve seed set in autotetraploidy breeding.

Cytological studies of a dihaploid and hybrid from intergeneric cross Elymus shandongensis × Triticum aestivum

One dihaploid (n=2x=14) and one true hybrid (2n=5x=35) were produced from the intergeneric cross Elymus shandongensis X Triticum aestivum. The dihaploid was a result of selective elimination of T. aestivum chromosomes early in embryo development. Meiotic chromosome behaviour was studied in the parents, dihaploid, and hybrid. E. shandongensis (2n=4x=28) and the hexaploid wheat formed only bivalents at metaphase I. Some homoeologous chromosome pairing with an average of 0.68 bivalents, ranging from zero to four, and a very low frequency of trivalents was observed in the dihaploid. This reveals a low affinity between S and Y genomes of E. shandongensis. A haplo-insufficient (hemizygous ineffective) genetic control of homoeologous chromosome pairing may also exist in the species. Homologous chromosome association in the hybrid (0.25 chiasmata/cell) was distinctly lower than in the dihaploid (0.73 chiasmata/cell). This indicates that chromosome pairing suppressors, e.g., the Ph gene system, in T. aestivum could affect homoeologous chromosome pairing between the genomes of wild species.

Morphological, yield, cytological and molecular characterization of a bread wheat X tritordeum F1 hybrid

The morphological, yield, cytological and molecular characteristics of bread wheat x tritordeum F(1) hybrids (2n = 6x = 42; AABBDH(ch)) and their parents were analysed. Morphologically, these hybrids resembled the wheat parent. They were slightly bigger than both parents, had more spikelets per spike, and tillered more profusely. The hybrids are self-fertile but a reduction of average values of yield parameters was observed. For the cytological approach we used a double-target fluorescence in situ hybridization performed with total genomic DNA from Hordeum chilense L. and the ribosomal sequence pTa71. This technique allowed us to confirm the hybrid nature and to analyse chromosome pairing in this material. Our results showed that the expected complete homologous pairing (14 bivalents plus 14 univalents) was only observed in 9.59% of the pollen mother cells (PMCs) analysed. Some PMCs presented autosyndetic pairing of H(ch) and A, B or D chromosomes. The average number of univalents was higher in the wheat genome (6.8) than in the H(ch) genome (5.4). The maximum number of univalents per PMC was 20. We only observed wheat multivalents (one per PMC) but the frequency of trivalents (0.08) was higher than that of quadrivalents (0.058). We amplified 50 RAPD bands polymorphic between the F(1) hybrid and one of its parents, and 31 ISSR polymorphic bands. Both sets of markers proved to be reliable for DNA fingerprinting. The complementary use of morphological and yield analysis, molecular cytogenetic techniques and molecular markers allowed a more accurate evaluation and characterization of the hybrids analysed here.

Meiotic chromosome behaviour in an allotriploid derived from an amphidiploid × diploid mating in Cucumis

AbstractA synthetic amphidiploid species Cucumis hyriviis Chen & Kirkbride (2n = 4x = 38; genome designated as HHCC) has recently been created from an interspecific mating between C. sativus L. (2n = 2x = 14: genome designated as CC) and C. hystrx. Chakr. (2n = 2x = 24; genome designated as HH). This amphidiploid is resistant to root knot nematode, tolerant to low irradiance, and has higher nutritional value than standard processing cucumber cultivars. An allotriploid (2n = 3x = 26; HCC) was derived from a cross between C. sativus L. and C. hytivus Chen & Kirkbride. Diploid meiotic behaviour in C. sativus and C. hystrix involves the development of seven and 12 metaphase bivalents respectively. In the derived allotriploid. univalents. bivalents, and trivalents (at relatively low frequency) were observed at metaphase I indicating that some homeologues from the C and H genomes can synapse. Based on a comparative karyotype analysis of cucumber (i.e. chromosome size and pairing behaviour) and aliotriploid plants, the seven bivalents observed at metaphase I were ascertained to be cucumber homologues, while the 12 univalents were of C. hystrix origin thus confirming the allotriploid karyotypic constitution to be HCC. On average, the frequency of trivalents was 0.24 at diakinesis and 0.22 in 100 meiocytes at metaphase I. indicating the possibility of genetic exchange due to the homoeology between the C and H genomes. After simultaneous cytokinesis, only polyads were observed in pollen mother cells (PMCs) at telophase II, which led to the production of sterile pollen grains. Multi‐polarization of chromosomes was dominant at anaphase II. However. in about 20% PMCs. chromosomes separated to form a 7C + 1‐2H complement, suggesting a possible method for the production of alien addition cucumber‐C hystrix lines through further backcrossing of the allotriploid to diploid cucumber.

Cytogenetic Studies on Brassica Campestris Primary Trisomics

Meiosis, trisomic transmission, and inheritance of erucic acid content were studied in a nucleolar trisomic (No. 2891) and a non-nucleolar trisomic (No. 2881) of Brassica campestris. The two trisomics originated from a B. campestris-alboglabra monosomic addition line whose alien chromosome carried the gene for erucic acid content. Meiotic analyses revealed that the trivalent frequencies in the pollen mother cells (PMCs) at diakinesis/metaphase I were 33.3% and 20% in No. 2891 and No. 2881, respectively. The transmission rates of the extra chromosome through the ovule in No. 2891 and No. 2881 were about 30.6% and 32.4%, respectively. The inheritance of erucic acid content was investigated in the two crosses, No. 2891 ×B. campestris landrace “Wu Chang Bei You Cai” and No. 2881 ×B. campestris landrace “Xi Shui You Cai Bei”. The segregation of erucic acid content in the F2 progenies of the selfed trisomic F1 plants from the two crosses showed a disomic inheritance pattern, thus indicating that the trisomy in No. 2891 and No. 2881 did not involve the chromosome carrying the erucic acid gene. The genetic divergence of homoeologous chromosomes between B. campestris and B. alboglabra is discussed.

Meiotic behaviour of individual chromosomes in allotriploid Alstroemeria hybrids.

Chromosome association and chiasma formation were studied in pollen mother cells at metaphase I of four allotriplod BC1 plants (2n=3x=24) obtained from the backcross of the hybrid Alstroemeria aurea x A. inodora with its parent A. inodora. We distinguished the chromosomes of both parental species by genomic in situ hybridization (GISH), whereas the individual chromosomes were identified on the basis of their multicolour FISH banding patterns obtained after a second hybridization with two species-specific satellite repeats as probes. All the four BC1 plants possessed two genomes of A. inodora and one of A. aurea. Variable numbers of recombinant chromosomes, resulting from meiotic recombination in the interspecific hybrid, were present in these plants. The homologous A. inodora chromosomes generally formed bivalents, leaving the homoeologous A. aurea chromosomes unassociated. High frequencies of trivalents were observed for the chromosome sets that contained recombinant chromosomes, even when the recombinant segments were small. Chromosome associations in the trivalents were restricted to homologous segments. The implications of the absence of homoeologous chromosome pairing on gamete constitution and prospects for introgression in Alstroemeria are discussed.

Interspecific crossability and cytogenetic analysis of sexual progenies of Mexican wild diploid 1EBN speciesSolanum pinnatisectum andS. cardiophyllum

Mexican wild diploid species,Solanum pinnatisectum (S. pnt) (2n=2x=24, 1EBN: endosperm balance number) is a useful germplasm source of late blight and Colorado potato beetle (CPB) resistance in potato improvement. However, it is very difficult to cross this species with other 1EBNSolanum species. Sexual hybrids among three accessions ofS. pnt and two accessions ofS. cardiophyllum (S. cph) (2n=2x=24, 1EBN) were studied. There were large differences in the cross-compatibility among the genotypes and accessions from these two 2x-1EBN species. Interspecific incompatibility existed in the crosses betweenS. pnt andS. cph, in whichS. cph functioned only as the male parent. The crosses with accessions PI 275236 ofS. pnt produced 35 hybrids following the conventional propagation procedures. Seven hybrids were obtained with theS. pnt accession PI 275233 with the aid of embryo rescue, while hybridization involving accession PI 253214 resulted in no fruit. The cytogenetic analysis indicated that all of the hybrids derived from crosses of accession PI 275233 ofS. pnt withS. cph had 2n = 24 chromosomes and were pollen fertile. Crosses involving accession PI 275236 ofS. pnt produced both 2x (2n=24) and 3x (2n=36) hybrids. Meiotic analysis at metaphase I of pollen mother cells (PMCs) showed a fairly high rate of chromosome pairing that averaged between 10.10 and 11.10 bivalents per cell in most of the 2x hybrids, indicatingS. pnt andS. cph have the similar genome. A high frequency of trivalents and quadrivalents were observed in 3x hybrids derived fromS. pnt 2 ×S. cph 1, indicating homology exists among these chromosomes. These results suggest that specific gene(s) and not either differences between genomes or EBN controlled interspecific crossability and embryo development. The disease and insect tests on the 2x and 3x hybrids revealed that all have high levels of resistance to both late blight and CPB. Thus it could appear that selection of genotypes is a key for successful interspecific hybridization when using Mexican wild diploid species as a source of economic important traits.

Sex-dependent synaptic behaviour in triploid turbot, Scophthalmus maximus (Pisces, Scophthalmidae).

A surface-spreading synaptonemal complex (SC) technique was used to analyse spermatocytes and oocytes of triploid turbot (Scophthalmus maximus) in order to visualise the process of chromosome synapsis. The most conspicuous characteristic of triploid oocytes is that, in the trivalents, the lateral elements of the SC were frequently associated in threes, either completely along the length of the trivalent, or partially, forming a variety of forked structures. In these nuclei, synapsis usually occurred among homologous chromosomes and the number of bivalents observed was significantly higher than that expected under the assumption of random chromosome association among all partners. However, the frequency of trivalents was very low in triploid spermatocytes, triple synapsis being also scarce. In these nuclei chromosomes that were excluded from homologous synapsis become engaged in random SC formation, and, therefore a considerable number of non-homologous associations are produced. The causes of the synaptic differences observed in triploid males and females of turbot and their possible relation to the sterility displayed by these animals are discussed.

Spermatogenic failure in male mice with four sex chromosomes.

There is accumulating evidence that meiosis, like mitosis, is monitored by a number of checkpoints. In mammals, the presence of asynapsed chromosomes at pachytene triggers a checkpoint (the pachytene or synapsis checkpoint) that removes cells via a p53-independent apoptotic pathway. In the special case of the sex bivalent in males, it is pseudoautosomal region (PAR) asynapsis that triggers the checkpoint. In male mice with three sex chromosomes (XYY or XYY(*X)) some pachytene spermatocytes achieve full (trivalent) PAR synapsis, but in many cells one sex chromosome remains as a univalent, thus triggering the checkpoint. Sperm counts in these males have been shown to be positively correlated with trivalent frequencies. In the present study sperm production and levels of sex chromosome synapsis were studied in mice with four sex chromosomes (XYYY(*X)) and XYY(*X)Y(*X)). These mice proved to be more severely affected than XYY or XYY(*X) mice. Nevertheless, pachytene synaptonemal complex analysis revealed that full PAR synapsis was achieved through the formation of radial quadrivalents or through the formation of two sex bivalents in 21%-49% of cells analysed. Given these levels of full PAR synapsis, the sperm counts were consistently lower than would have been predicted from the relationship between levels of PAR synapsis and sperm counts in mice with three sex chromosomes. It has been suggested that the inactivation of the asynapsed non-PAR X and Y axes of the XY bivalent of normal males (MSCI), which occurs during meiotic prophase, may be driven by Xist transcripts originating from the X. If this is the case, the non-PAR Y axes of YY and YY(*X) bivalents would fail to undergo MSCI. This could be cell lethal, either because of 'inappropriate' Y gene expression, or because the non-PAR Y axis may now trigger the synapsis checkpoint.

Meiosis in primary trisomics of rye: considerations for models of chromosome pairing.

Meiotic chromosome pairing of primary trisomics of rye was analysed by electron microscopy in surface-spread prophase I nuclei and compared with light-microscopic observations of metaphase I cells. Despite the large-sized chromosomes of rye, prophase I trivalent frequencies were close to the two thirds expected on a simple model with two terminal independent pairing initiation sites per trisome (set of three homologous chromosomes). Direct observations mostly reveal one pairing partner switch (PPS) in prophase I trivalents, which confirms this supposition. There were no significant differences between the number of trivalent and bivalent plus univalent configurations observed at prophase and metaphase I; therefore, synapsed segments form chiasmata. In all of the trisomics, the three homologues showed variations not only in the number of telomeric C-bands but also in the amount of heterochromatin of these bands, which allowed identification of chromosomes or chromosome arms associated in most metaphase I configurations. In trisomics for chromosomes 2, 3 and 5, some metaphase I chromosome configuration frequencies did not fit those expected under the assumption of random chromosome association among all partners, suggesting the existence of preferences for pairing between two given chromosome arms of the trisome. No preferential associations either at metaphase I or pachytene were observed in the trisomics for chromosome 6. The fit between theoretical pairing models and the experimental data is also discussed.

Evidence that sex chromosome asynapsis, rather than excess Y gene dosage, is responsible for the meiotic impairment of XYY mice

There is extensive evidence for the existence of a meiotic checkpoint that acts to eliminate spermatocytes that fail to achieve full sex chromosome synapsis at the pachytene stage of the first meiotic prophase. XYY mice are nearly always sterile, with clear signs of meiotic impairment, and sex chromosome asynapsis has been proposed to underlie this impairment. However, a study of XYY*^X mice (mice having three sex chromosomes but only a single dose of Y genes) revealed that these mice are fertile, and thus implicated Y gene dosage as a major factor in the sterility of XYY mice. To address this question further, sex chromosome synapsis and spermatogenic proficiency were compared between XYY*^X and XYY mice generated in the same litters. This established that differences in spermatogenic proficiency within and between the two genotypes correlated with the frequency of radial trivalent formation (full sex chromosome synapsis); XYY*^X males, as a group, had double the radial trivalent frequency of XYY males. This observation provides strong support for the view that sex chromosome asynapsis (or some consequence thereof), rather than Y gene dosage, is the major factor leading to the meiotic impairment of XYY mice.

Characterization of sexual progenies of male-sterile somatic cybrids between Brassica napus and Brassica tournefortii

Cytogenetic studies were performed on four male-sterile progenies derived from four different cybrids produced between Brassica napus and B. tournefortii using the donor-recipient protoplast fusion method. The objective of these studies was to characterize the nuclear constitution of the plants. Mitotic investigation revealed that three of the four male-sterile lines had 38 chromosomes, which is equal to that of B. napus. The fourth line, C6, had variable chromosome numbers, ranging from 39 to 42 in different plants. The meiotic behavior in each progeny varied distinctly. Of the plants having 38 chromosomes, fairly high chromosome pairing, on average 18.08 bivalents per cell, was detected at metaphase-I. However, univalents with an average of 1.39 per cell, and very low frequencies of trivalents and/or tetravalents, were also observed in the lines. These results revealed that male-sterile cybrid lines were obtained with 38 chromosomes and a relatively high level of chromosome-pairing ability, indicating their potential for establishing a stable male-sterile rapeseed line.

Meiosis in Triploid Lolium. IV. Distribution of Chiasmata in Autotriploids.

The genome of Lolium temulentum has 50% more DNA than the L. multiflorum genome. However, chiasma and trivalent frequencies are not significantly different between the autotriploids of the species but there are differences in the frequencies of the various types of trivalent. The data show that, irrespective of genome size, each set of three homologous chromosome arms is first allocated a chiasma ensuring a ring bivalent or chain trivalent then additional chiasmata convert either of these configurations into a frying pan or bird cage trivalent.

Cytogenetics of Eurasian Bluebunch Wheatgrass and Their Relationship to North American Bluebunch and Thickspike Wheatgrasses

Bluebunch wheatgrass, Pseudoroegneria spicata (Pursh.) A. Love, is one of the most important native dryland forage bunchgrasses of the intermountain West, which often produces depleted stands under moderate to heavy grazing pressure. In recent plant collecting trips to the Altai and Tien Shan Mountains of China and Russia, a grazing tolerant bluebunch wheatgrass complex was obtained. This study was conducted to describe the cytogenetic characteristics and potential for genetic exchange between the Eurasian bluebunch wheatgrass complex [P. strigosa (M. Bieb.) A. Love, P. geniculata (Trin.) A. Love, Roegneria elytrigioides Yen and Yang, and R. glaberrima Keng et S.L. Chen] and those bluebunch thickspike wheatgrasses found in North America. Meiotic behavior of the triploid hybrids between diploid and tetraploid R. glaberrima indicated that both ploidy levels share the same genome. Almost complete pairing in hybrids P. strigosa ssp. aegilopoides (Drob.) A. Love × P. spicata, and R. glaberrima × P. spicata, demonstrated that P. strigosa ssp. aegilopoides and R. glaberrima are comprised of the basic S genome. Triploid hybrids between P. geniculata, and R. elytrigioides with either P. spicata, P. stipifolia (Czern. ex Nevski) A. Love, or P. strigosa ssp. aegilopoides exhibited trivalent frequencies consistent with the hypothesis that tetraploid R. elytrigioides is an autotetraploid (SSSS) and that P. geniculata is either a segmental autotetraploid (S1S1S2S2) or an allotetraploid (SS??). Based on the high level of chromosome pairing in the interspecific hybrids between the Eurasian and North American bluebunch wbeatgrass complex, genetic exchange is possible. Due to the close genetic and morphological similarity among individuals of P. spicata, P. strigosa, P. geniculata, R. elytrigioides, and R. glaberrima we are likely dealing with a single, polymorphic, Eurasian‐North American species complex.

Preferential pairing estimates from multivalent frequencies in tetraploids

Mathematical models are presented for estimating preferential pairing and chiasma parameters in amphidiploids and autotetraploids on the basis of diakinesis or metaphase I configuration frequencies and are compared with other approaches of estimating affinity. With a preferential pairing factor p, estimated from quadrivalent and trivalent frequencies, and estimated chiasmate association factors for the two arms in quadrivalents (a(qu) and b(qu) for arms A and B, respectively) and in bivalents (a(bi) and b(bi)) a perfect fit between observed and predicted configuration frequencies can often be obtained in amphidiploids of several plant species, including Solanaceae and Gramineae. Since several proven autotetraploids give very similar apparent preferential pairing estimates, the biological significance of such parameters as preferential pairing and affinity factors is considered limited. The same is true for pairing parameters estimated by optimizing fit of configuration frequencies expected on the basis of theoretical models to observed data.

Production and identification of primary trisomics in diploid Agropyron cristatum (crested wheatgrass)

Six of the seven possible primary trisomics in Agropyron cristatum were produced. Based on morphology, arm length ratios, and C-banding patterns, they were identified as primary trisomics for chromosomes A, B, C, D, E, and G. Agropyron cristatum is one of several species constituting the crested wheatgrass complex. All species in this complex contain one basic genome (P). A study was conducted to produce and identify a primary trisomic series that will be used to map genes to individual chromosomes. A population of 157 plants were generated by crossing autotriploids (PPP) with diploid (PP) A. cristatum: 58 were diploid (2n = 14), 76 were primary trisomies (2n = 15), 17 were double trisomic (2n = 16), 4 were triple trisomics (2n = 14 + 3), 1 was telocentric trisomic (2n = 14 + 1 telo), and 1 was tetratrisomic (2n = 14 + 4). Karyotype analysis of acetoorcein-stained chromosomes was carried out using the CHROMPAC III computer program; for analysis of C-banded karyotypes, the computer imaging analysis program PCAS (Plant Chromosome Analysis System) was used to identify the primary trisomics. Of the 47 primary trisomics analyzed, 21 plants had one extra satellited chromosome E, 18 with the satellited D chromosome, 3 each for chromosomes B and G, and 1 each for chromosomes C and A. Chromosome pairing was studied in trisomies B, D, E, and G. Trisomics for chromosomes B and G were similar in their mieotic behavior. Each had a trivalent frequency of about 60% and pollen stainability of less than 40%. Trisomics for chromosomes D and E had a trivalent frequency of about 30% and pollen stainability of over 70%.

Potato germ plasm enhancement with disomic tetraploid Solanum acaule. II. Assessment of breeding value of tetraploid F1 hybrids between tetrasomic tetraploid S. tuberosum and S. acaule

The breeding value of tetraploid F1 hybrids between tetrasomic tetraploid S. tuberosum and the disomic tetraploid wild species S. acaule was examined. The F1 hybrids showed a tuber yield and appearance comparable to those of their cultivated parent, indicating a potential as acceptable breeding stocks despite the 50% contribution to their pedigree from wild S. acaule. The cytological behavior of the tetraploid F1 hybrids was examined to determine the probability of recombination for the introgression of S. acaule genes. The majority of the meiotic configurations at metaphase I was bivalents and univalents with mean frequencies of 17.6 and 9.9, respectively. Further, a low frequency of trivalents and quadrivalents was observed. An acceptable low level of meiotic irregularities were observed at the later stages of microsporogenesis, and a reasonable level of pollen stainability was obtained. Therefore, these hybrids could likely be employed for further introgression. From the cytological observations, the following speculations were drawn: (1) some genomic differentiation exists between the S. acaule genomes, (2) at least one of the S. acaule genomes may be homoeologous to the S. tuberosum genomes, (3) intergenomic recombination would likely occur due to the nature of the genomic constitution of the hybrids, and (4) the nature of sesquiploidy of the hybrids may facilitate efficient introgression and establishment of unique aneuploid and euploid recombinant genetic stocks.

Cytological studies on meiotic configurations of intraspecific aneuploids ofCarex blepharicarpa (Cyperaceae) in Japan

Chromosome configurations at meiotic metaphase I ofCarex blepharicarpa were determined for 245 individuals collected from 11 localities in the Chugoku District of Japan. Nine intraspecific aneuploids, 2n=26–33 and 41, were found. The most common diploid number was 29, and found in 73 individuals. No clear geographical pattern was suggested by a distribution of these aneuploids. A consecutive series of chromosome numbers from 2n=26 to 32 was found in two populations from Okayama Prefecture. At meiotic metaphase I, univalents and multivalents were found, and one to three trivalents were observed in each aneuploid. Heteromorphic chain trivalents comprising large, medium and small chromosomes were prevalent. Homomorphic trivalents, which are thought to be originated from chromosome duplications caused by unreduced gametes, were found in only one individual with 2n=41. The high frequency of heteromorphic trivalents in this species indicates that most aneuploidy probably results from fission and/or fusion of chromosomes.

Synaptic patterns of rye B chromosomes. I: The standard type.

Chromosome pairing of standard B chromosomes (Bs) in rye has been studied by synaptonemal complex surface spreading of pollen mother cells containing from one to eight Bs. The main characteristics exhibited by the Bs are: (i) They are often located peripherally in surface spread nuclei and do not show presynaptic alignment. (ii) The period of pairing is delayed relative to that of the A set. (iii) Bs undergo extensive non-homol-ogous pairing at pachytene. (iv) The frequency of trivalents and quadrivalents in plants with 3Bs and 4Bs is about 66% following the expectations on the basis of random pairing among the short and the long arms with a single distal pairing initiation site per arm. Some quadrivalents containing two partner switches were observed, but on the same basis of randomness there was a deficiency of pachytene multivalents in plants with 5-8 Bs. The possible causes of the discrepancy between these observations are discussed. (v) There is a reduction in the frequency of multivalents between pachytene and metaphase I.

Chromosome Transmission and Meiotic Stability of Sugarcane (Saccharum spp.) Hybrid Derivatives

Efficiency of selection in sugarcane (Saccharum spp. hybrids) breeding may be influenced by the type of chromosome transmission. Our objectives were to determine chromosome transmission and meiotic stability among and within sugarcane families derived from crosses of elite, interspecific clones with four S. spontaneum L. clones. The cytological behavior, pollen stainability, and seed yield were measured for 23 nonrecurrent clones in parental, F1, and BC1 to BC3 generations. Chromosome number was determined for 13 recurrent parents. Chromosome transmission was strictly n + n, although meiotic irregularity, aneuploidy, and chromosomal mosaicism resulted in aneuploid gametes. Chromosome number increased quadratically with generations from 2n=60.5 in S. spontaneum parents to 2n=84.0 at F1, 97.5 at BC1, 106.1 at BC2, and 108.0 at BC3. Chromosomes paired primarily as bivalents, although variable numbers of univalents and multivalents were observed. The frequency of trivalents increased linearly with generations, and was greater in hybrid derivatives than in parental S. spontaneum clones. Multivalent pairing indicated that autosyndesis was incomplete. Meiotic irregularities also included multiple nucleoli, anaphase bridges and laggards, asynchronous meiosis, chromosome fragments, pseudobivalents, first and second division restitution nuclei, and syncytes. One clone, ‘CP 33‐224’, was partially desynaptic. Results suggest that sugarcane cultivars and S. spontaneum share a common endosperm balance number unlike S. officinarum L. and S. spontaneum. Transmission by n + n may indicate that additional cycles of random mating among hybrid derivatives, relative to that with 2n + n transmission, may be necessary to recover desirable recombinants in sugarcane populations.