The karyotype of Nodipecten nodosus (Bivalvia: Pectinidae)

Earlier karyotypical work on Nodipecten nodosus embryos indicated that this species has a diploid number of 38, with six pairs respectively of metacentric and submetacentric chromosomes and seven pairs of subtelocentric chromosomes, although there were some difficulties in obtaining complete metaphases. The present work provides additional results on specific regions of the chromosomes in N. nodosus and, by meiotic studies, confirms the chromosome number with more reliability. Active nucleolar organizer regions (NOR), detected in mitotic metaphases from embryos, can be characterized in N. nodosus by a high level of heteromorphism of NOR-sites, indicating that these regions are not appropriate as chromosomal markers in this species. The procedure for detecting constitutive heterochromatin of chromosomes allowed us to observe most of the heterochromatic blocks at a pericentromeric position and some at telomeric and interstitial positions. The analysis of meiotic chromosomes from gonad tissue revealed the presence of 19 bivalents during metaphase I, all homomorphic and isopicnotic, confirming the previously described diploid chromosomal number of 38 for N. nodosus. From these results, some evolutionary aspects of the Pectinidae are briefly discussed.     

The fate of multivalents during meiotic prophase in the hybrid Gibasis consobrina x G. karwinskyana Rafin. (Commelinaceae)

The chromosomes of the two closely related diploid species, Gibasis consobrina and G. karwinskyana (Commelinaceae; 2n=2x=10), are morphologically alike, yet form few chiasmate associations at metaphase I in the f₁ hybrid. During meiotic prophase, however, synaptonemal complexes join the majority of the chromosomes of the complement in complex multiple pairing configurations. The F₁ hybrid between different tetraploid genotypes of the same two species similarly forms multivalents during meiotic prophase, which are subsequently eliminated in favour of strictly homologous bivalents before metaphase I. One quadrivalent comprising interchange chromosomes inherited from one of the parents, usually persists to first metaphase. Evidently the resolution of multivalents to bivalents at first metaphase, which accounts for diploidisation, is not attributable to the elimination of multivalents per se, but of multivalents comprising chromosomes of limited homology.     

Chromosome differentiation in diploid species of Lotus (Leguminosae)

Summary Meiotic chromosome behavior was studied in seven diploid species of Lotus (L. alpinus Schleich., L. japonicus (Regel) Larsen, L. filicaulis Dur., L. schoelleri Schweinf., L. krylovii Schischk. and Serg., L. tenuis Waldst. et Kit., L. corniculatus var. minor Baker) and in 51 interspecific hybrids from 16 different crosses. Meiosis in the diploid species was quite regular. In a high proportion of the PMC's of the hybrids there was close chromosome homology with a normal association of 6 II's. However, meiotic irregularities including bridges, lagging chromosomes, univalents, and quadrivalents, occurred in a small percentage of the cells. The late separation of bivalents, the presence of quadrivalents, and inversion bridges with fragments, would indicate for some hybrids that certain chromosomes were structurally differentiated. The large number of rod bivalents observed at diakinesis was also highly suggestive that genetic nonhomology in one chromosome arm could contribute to the frequency of this type of bivalent. Therefore, the maximum number of 6 II's which occurred in a high percentage of cells may be misleading in that cryptic structural differences between chromosome arms, or segments, are not revealed. Pollen fertility in the species and hybrids was not correlated with meiotic irregularities suggesting that pollen fertility is genotypically controlled.     

Cytogenetic studies on <Emphasis Type="Italic">Metasequoia glyptostroboides</Emphasis>, a living fossil species

The chromosome morphology and meiotic pairing behavior in the pollen mother cells (PMCs) of Metasequoia glyptostroboides were investigated. The results showed that: (1) The chromosome number of the PMCs was 2n=22. (2) The PMCs developed in the successive manner, and the nucleoids in the dynamic development were similar to those of the other gymnosperms. (3) At prophase, most of the chromosomes were unable to be identified distinctively because the chromosomes were long and tangled together. The chromosome segments were paired non-synchronously. At pachytene, the interstitial or terminal regions of some bivalents did not form synapsis and the paired chromosomes showed difference in sizes, indicating that there were structure differences between the homologous chromosomes. (4) At diakinesis, the ring bivalents showed complicated configurations due to the differences in location and number of chiasmata. In addition, there were cross-linked bivalents. (5) At metaphase I, the chromosome configuration of each cell was 8.2II ⁰ + 1.1II + 1.3II ⁺ + 0.8I. Most of the chromosomes were ring bivalents, but some were cross-linked bivalents, rod bivalents, or univalents. (6) 15\% PMCs at anaphase I and 22\% PMCs at anaphase II presented chromosome bridges, chromosome fragments, micronuclei, and lagging chromosomes. Twenty seven percent microspores finally moved into one to three micronuclei. Twenty five percent pollens were abortive. The results indicated that the observed individual of M. glyptostroboideswas probably a parpcentric inversion heterozygote, and there were structural and behavioral differences between the homologous chromosomes. The chromosomal aberration of M. glyptostroboidesmay play an important role in the evolution of this relict species, which is known as a living fossil. Further evidence is needed to test whether the differences between homologous chromosomes were due to hybridization.     

Introgression of chromosomes of Festuca arundinacea var. glaucescens into Lolium multiflorum revealed by genomic in situ hybridisation (GISH)

An F₁ hybrid (n=4x=28) between the tetraploid species Festuca arundinacea var. glaucescens (GGG′G′) and a synthetic tetraploid Lolium multiflorum (LmLmLmLm) was backcrossed to diploid L. multiflorum to produce triploid (2n=3x=21) BC₁ hybrids (LmLmG). At metaphase I of meiosis the triploids had a preponderance of ring bivalents and univalents with some linear and frying-pan trivalents. Genomic in situ hybridisation (GISH) differentiated the Festuca chromosomes from Lolium and revealed that the bivalents were exclusively between Lolium homologues, while the univalents were Festuca. Despite the limited amount of homoeologous chiasmata pairing in the triploids, some recombinant chromosomes were recovered in the second backcross when the hybrids were further crossed to diploid L. multiflorum. The progeny from the second backcross was predominantly diploid. Genotypes with recombinant chromosomes and chromosome additions involving an extra Festuca chromosome were identified using GISH. Changes in plant phenotype were related to the presence of Festuca chromatin. Received: 20 September 2000 / Accepted: 05 January 2001     

Chromosome number and secondary chromosomal associations in wild populations of Geranium pratense L. from the cold deserts of Lahaul-Spiti (India)

In this work we studied the meiotic chromosome number and details of secondary chromosomal associations recorded for the first time in Geranium pratense L. from the alpine environments in the cold deserts of Lahaul-Spiti (India). All the presently studied individuals of the species existed at 4x level (x = 14). The present chromosome count of n = 28 in the species adds a new cytotype to the already existing diploid chromosome count of 2n = 28 from the Eastern Himalayas and outside of India. Out of the six accessions scored presently four showed normal meiotic course. However, two accessions investigated from Mud, 3800 m and Koksar, 3140 m depicted abnormal meiotic course due to the presence of multivalents and univalents, and secondary associations of bivalents/chromosomes. The secondary chromosomal associations in the species existed among bivalents/chromosomes were noticed in the PMCs at prophase-I (diakinesis) and persisted till the separation of sister chromatids at M-II. The variation in the number of bivalents/chromosomes involved in the secondary associations at M-I (2–8) and A-I/M-II (2–12) has also been recorded. The occurrence of such secondary associations of bivalents/chromosomes in G. pratense which existed at 4x level indicated the secondary polyploid nature of the species.     

CHROMOSOME NUMBERS IN CHEILANTHES AND POLYPODIUM

Six chromosome observations are reported, two being confirmations and four being new counts. Polypodium plesiosorum (from Mexico) shows 37 pairs at meiotic metaphase and thus conforms to past counts of diploid species in this genus. Cheilanthes wrightii (Arizona) has 2n = 58 and is a diploid member of the x = 29 series of that genus. Cheilanthes tomentosa (Alabama) shows 90 units at metaphase of meiosis and approximately the same number during mitosis in the crozier cells. It has 32 spores and our plant is an apogamous triploid. Both Cheilanthes siliquosa (Washington) and C. californica (California) show 30 pairs at meiotic metaphase. Counts on plants identified as C. carlotta-halliae (California) show 30 bivalents and 30 univalents at diakinesis, suggesting that they actually represent allopolyploid backcrosses of C. carlotta-halliae and one of its parents.     

Meiosis in Drosophila melanogaster

Individual bivalents or chromosomes have been identified in Drosophila melanogaster spermatocytes at metaphase I, anaphase I, metaphase II and anaphase II in electron micrographs of serial sections. Identification was based on a combination of chromosome volume analysis, bivalent topology, and kinetochore position. — Kinetochore microtubule numbers have been obtained for the identified chromosomes at all four meiotic stages. Average numbers in D. melanogaster are relatively low compared to reported numbers of other higher eukaryotes. There are no differences in kinetochore microtubule numbers within a stage despite a large (approximately tenfold) difference in chromosome volume between the largest and the smallest chromosome. A comparison between the two meiotic metaphases (metaphase I and metaphase II) reveals that metaphase I kinetochores possess twice as many microtubules as metaphase II kinetochores. — Other microtubules in addition to those that end on or penetrate the kinetochore are found in the vicinity of the kinetochore. These microtubules penetrate the chromosome rather than the kinetochore proper and are more numerous at metaphase I than at the other division stages.     

Chromosomes of Aspidogaster conchicola

Analysis of mitotic and meiotic chromosome spreads of Aspidogaster conchicola (Trematoda: Aspidogastrea) reveals a diploid number of 10 for this species. Haploid sets containing 5 bivalents confirm the diploid number. The karyotype consists of one pair of comparatively long subtelocentric chromosomes and four pairs of shorter acrocentric elements. The results are discussed in comparison with existing data on chromosomes of aspidogastrean species and other related helminth groups.     

Chromosome systems in the eriococcidae (Coccoidea Homoptera)

Spermatogenesis is described in two eriococcid species and the observations are compared to those previously reported. In Gossyparia spuria the diploid chromosome number is 28 in both males and females. In the female all the chromosomes are euchromatic. In most male tissues 14 of the chromosomes are euchromatic (E) and 14 are heterochromatic (H). Prior to the first meiotic division in males the number of H chromosomes was reduced. During prophase I all the cells showed 14 E chromosomes and from 1 to over 9 H chromosomes. The range of chromosome numbers in metaphase I was similar to that in prophase I. All the chromosomes divided in anaphase I, and, following differential uncoiling at interkinesis, the E and H groups of chromosomes segregated from each other at anaphase II. Only the E groups formed sperm. The presence of a variable number of H chromosomes and a haploid number of E chromosomes in spermatogenesis suggested the presence of the multiple-D variant of the Comstockiella chromosome system. In this system some of the H chromosomes become euchromatic prior to prophase I of spermatogenesis and pair with their E homologues. All the remaining H chromosomes are thus univalents, while among the E elements, some are univalents and the rest are bivalents. The observed reduction in the number of H chromosomes in the first meiotic division which was previously attributed to pairing among the H chromosomes, is now interpreted to be the result of the return of some of the H chromosomes to a euchromatic state and to their subsequent pairing with their E homologues. Spermatogenesis in Eriococcus araucariae was similar to that of G. spuria except that the reduction in the number of H chromosomes was not as extensive. The chromosome systems of the two species are compared to those of other eriococcids and the differences are briefly discussed.Supported by grant GB1585 from the National Science Foundation, Washington, D. C.     

Testicular chromosomes of Gallus domesticus

Summary Testes of Gallus domesticus were studied (a) by light microscopy after hypotonic treatment followed by acetic-alcohol fixation and airdrying and (b) by electron microscopy of osmium-fixed, araldite-embedded material, some of which was pretreated with hypotonic solutions.The following conclusions were reached: (i) The number of chromosome pairs at meiosis is constant and is most probably 40 (although 39 or 38 is possible). (ii) The diploid chromosome number at mitotic metaphase cannot be certainly determined by light microscopy but there is no reason to suppose it is not double the number of meiotic bivalents. (iii) No essential difference in structure was found between long and short bivalents at meiosis by light or electron microscopy; the lengths of the bivalents at pachytene form a continuous series, (iv) Some short bivalents appear to contain less material per unit length than long ones; this could explain why these chromosomes cannot always be resolved by light microscopy when fully contracted. (v) So-called macro- and micro-chromosomes differ only in size, but not in behaviour, at mitosis and meiosis.     

Nuclear division in the red algae Chondria nidifica and Chondria tenuissima

Cytological investigations are reported for two Chondria species, the Pacific species Chondria nidifica Harvey and Chondria tenuissima (Goodenough et Woodward) C. A. Agardh from the shore of the Marmara Sea in Istanbul. Nuclear division during mitosis and meiosis has been followed in somatic cells and in tetrasporangial mother cells respectively of diploid tetrasporic plants. The spherical interphase nucleus stains densely, showing many chromatin granules. Mitotic nuclei in the apical groove show a large number of chromosomes at metaphase; the chromosome number has been estimated at diakinesis to be 40 in both C. nidifica and C. tenuissima. The meiotic nuclei of tetraspore mother cells in prophase contain several relatively large nucleolar-derivatives in both species. The nucleolar derivatives disappear completely before the chromosomes begin to differentiate. In meiotic prophase the tetraspore mother cell enlarges from its original diameter. The period of the second meiotic anaphase seems to be extremely short in comparison with other nuclear phases. When the chromosomes reach the poles, they spread and subsequently form a relatively compact mass at telophase. The spindle has not been observed in C. tenuissima. Photographs are presented of nucleoli and nucleolar-derivatives in mitotic and meiotic divisions.     

EXPERIMENTAL HYBRIDS IN EPILOBIUM (INCLUDING SECT. ZAUSCHNERIA) SPECIES WITH N = 15 (ONAGRACEAE)

Experimental hybrids involving the three diploid subspecies of Epilobium sect. Zauschneria formed 15 bivalents at meiotic metaphase I, as did experimental hybrids between the three other species of Epilobium (comprising sect. Cordylophorum) with n = 15. The gametic chromosome number of E. suffruticosum Nutt., n = 15, and its relationship to the other two species are reported for the first time. Although we have not obtained hybrids between the species of these two sections, their morphological similarities are impressive and they are surely closely related.     

Meiosis and pollen mitosis in diploid and triploid Colocasia antiquorum Schott.

The relationship between diploid and triploid forms of Colocasia antiquorum Schott. was assessed through comparative meiotic and pollen mitotic studies. Owing to poor spreading of the chromosomes of both materials, karyological observations on pachytene nuclei were limited to a few chromosomes. Among the two nucleolar chromosomes and a metacentric, telochromomere-bearing chromosome of the diploid, the latter and one of the nucleolar chromosomes characterized by a heteropycnotic short arm were identified in both bivalent and trivalent associations in the triploid. The homologues in these cases were homomorphic and intimately paired. Two types of heteromorphic bivalents exhibiting partial pairing of homomorphic segments were also recorded in the triploid. Among the 14 bivalents of the diploid at diakinesis, two were nucleolus-associated. In the triploid, chromosomal associations at diakinesis included trivalents (2 to 9), bivalents and univalents, and the chiasma frequency per paired chromosome was lower than in the diploids. In 21.6 percent of the PMCs at this stage intragenomic pairing of one or two chromosomes was observed. Post-diakinesis stages in the diploid were regular while in the triploid they were marked by various irregularities in a majority of the cells. However, fertility (stainability), size and divisional frequency of pollen in both materials were remarkably similar. Chromosome numbers in pollen nuclei in the triploid ranged from 8 to 25. Based on these data an autopolyploid origin for the triploid Colocasia and a lower base number than the gametic chromosome number for this genus are advanced.     

SYNTHETIC HYBRIDS OF NEW WORLD AND OLD WORLD AGROPYRONS. I. TETRAPLOID AGROPYRON SPICATUM × DIPLOID AGROPYRON CRISTATUM

Four interspecific hybrids of tetraploid A. spicatum × diploid A. cristatum ‘Fairway’ were obtained by controlled pollinations of emasculated and unemasculated spikes of A. spicatum. Most vegetative and spike characteristics of the hybrids were intermediate between those of the parent species. Tetraploid A. spicatum behaved cytologically as an autotetraploid, with mean chromosome associations of 0.09 I, 7.95 II, 0.03 III, and 2.98 IV being observed in 103 cells interpreted. The diploid A. cristatum was cytologically regular and formed 7 bivalents in 160 of 163 cells examined. Meiosis in the triploid hybrids was highly irregular, and these plants were completely sterile. Chromosomes of A. spicatum and A. cristatum differed sufficiently in size so that they could be distinguished in the hybrids. Seven bivalents and 7 univalents were formed in 90.5% of 262 cells interpreted at metaphase I. Mean chromosome associations of 6.87 I, 6.98 II, and 0.05 III were observed in the hybrids. Most chromosome pairing was due to autosyndesis of A. spicatum chromosomes, but A. cristatum chromosomes occasionally paired among themselves and with A. spicatum chromosomes. Tetraploid A. spicatum was considered to be an autotetraploid. On the basis of cytological evidence, A. cristatum, A. spicatum, and their interspecific hybrids were represented by genome formulae of AA, BBBB, and ABB, respectively.     

Quantitative analysis of diploid translocation heterozygotes: test of models and equations

Summary Equations have been derived for two different models of chromosome pairing and chiasmata distribution. The first model represents the normal condition and assumes complete synapsis of homologous bivalents and the arms of interchange quadrivalents. This is followed by a nonrandom distribution of chiasmata among bivalents and multivalents such that each bivalent or bivalent-equivalent always has at least one chiasma. Univalents occur only as part of a III, I configuration at diakinesis or metaphase I. The second model assumes that a hologenomic mutation is present in which all chromosomes of a genome are equally affected. Two different assumptions can be made for such a mutation, and both give the same results: (1) homologous or homoeologous chromosome arms may be randomly paired or unpaired, but synapsis always leads to a crossover; (2) homologous or homoeologous arms always pair, but chiasmata are randomly distributed among the arms. The meiotic configurations at diakinesis or metaphase I are the same for both assumptions. Meiotic configurations of normal diploid interchange heterozygotes show good agreement with numbers predicted by the equations for nonrandom chiasmata distribution among configurations. Inter-specific hybrids with supernumerary chromosomes produced meiotic configurations frequencies in agreement with predictions of equations for random chiasmata distribution, but a hybrid without supernumeraries fitted the nonrandom expectations.     

Cytology of meiosis in the triploid gynogenetic salamander Ambystoma tremblayi

Female meiosis was analyzed in the triploid gynogenetic salamander Ambystoma tremblayi to determine the mechanism by which a stable chromosome number is maintained in this unisexual species. Gross details of the reproductive cycle and the cytology of meiosis were analyzed in 20 specimens and 320 oocytes involving all stages from early diplotene to the beginning of anaphase II Ovulation apparently continues progressively involving a few oocytes at a time. Oocytes from the ovary contained chromosomes in diplotene, and diakinesis. The first metaphase was not observed since this stage occurs swiftly either immediately prior to or during ovulation. Oocytes in the most anterior region of the oviduct were in metaphase II, and those in the most posterior region were undergoing the beginning of anaphase II. Telophase II was not observed. Chromosome numbers obtained at all stages of prophase gave counts of approximately 42 bivalents, equivalent to the triploid somatic number known for this species. Similar numbers of dyads were obtained from metaphase II plates. This analysis supports earlier evidence suggesting that the triploid number of chromosomes in oocytes of A. tremblayi is doubled prior to meiosis, and the somatic number is later restored by two normal meiotic divisions.     

Meiosis in the male of Puto albicans (Coccoidea-Homoptera)

Cytologically, Puto has proved more primitive than its taxonomic position would indicate. In the single previously described example (Hughes-Schrader, 1944), an uncomplicated inverted meiotic sequence was described for the males. The present example, P. albicans, showed a significantly more primitive inverted sequence. Unlike the other examples reported for aphids and coccids, the chromatids of the dyads neither dissociated nor reassociated during interkinesis. Instead, they remained closely associated and interconnected by an unresolved terminal chiasmate attachment. At first metaphase, the spindle attachments were localized to a restricted region of the poleward surface of the chromatids. Localization of attachment during meiosis and close association of chromatids during interkinesis are both suggestive of similar but not identical conditions expected in ancestors with an uninverted meiotic sequence. A second species proved intermediate between P. albicans and that described by Hughes-Schrader in which a complete cycle of dissociation and reassociation occurred during interkinesis. In P. albicans the pachytene bivalents showed no structures suggestive of centric localizations. At their greatest condensation at first metaphase, the chromatids were clearly subdivided into half chromatids. Limited observations were made on chromosomes of other species of Puto and of Phenacoleachia zealandica, and also on spermiogenesis and mycetocyte formation in Puto. The discussion is devoted to considerations of chromatid subdivision, holokinetic chromosomes, and meiotic inversion and some evolutionary implications are mentioned.Supported by grant GB 4289 from the National Science Foundation.     

New Cases of Parthenogenesis and Polyploidy in the Genus Ramazzottius (Tardigrada,Hypsibiidae) and a Hypothesis Concerning Their Origin

Summary

Specimens of the genus Ramazzottius Binda and Pilato, 1986 (Eutar-digrada, Hypsibiidae) were obtained from 2 moss and 1 lichen sample(s) collected in the Emilian Apennine Mountains. R. tribulosus was only found in one sample, whereas R. oberhaeuseri was found in all three. The first species had only diploid specimens, with 6 bivalents during the first meiotic division; the second had only females showing various polyploid cytotypes in addition to the diploid bisexual cytotype cited for this area. One of the triploid and the tetraploid cytotypes were characterized by the presence of univalents at oocyte metaphase. In contrast, another cytotype had “bivalents” in triploid number. Though the large number of cytotypes found in a single sample may be attributed to chance, it is better explained by an in loco origin, at least in some cases.     

Meiotic studies in Acanonicus hahni (Stål) (Coreidae,Heteroptera) I. Behaviour of univalents in desynaptic individuals

Male meiosis was studied in a population of Acanonicus hahni (Stål), and nine of the sixteen individuals analyzed showed desynapsis. The frequency of univalents varied from one to seven percent in eight of them, while in the ninth the percentage of cells with univalents was higher (12%). The univalents auto-orientate at metaphase I in the center of the ring formed by autosomal bivalents and divide equationally at anaphase I; at metaphase II they show touch-and-go pairing, and lie in the center of the ring of autosomes.A desynaptic origin of the univalents is proposed, and the arrangement of the chromosomes in the first and second metaphase plate in the normal and desynaptic individuals is compared and discussed. The meiotic characteristics of these desynaptic individuals are also compared with those described in other insects with holocentric and monocentric chromosomes. It is suggested that any achiasmatic chromosome, whether a univalent, m or sex chromosome, will induce the formation of a ring and with some or all of them lying in its centre.