Kinds Of Chromosomes Research Articles

Similarity in replication timing between polytene and diploid cells is associated with the organization of the Drosophila genome.

Morphologically, polytene chromosomes of Drosophila melanogaster consist of compact “black” bands alternating with less compact “grey” bands and interbands. We developed a comprehensive approach that combines cytological mapping data of FlyBase-annotated genes and novel tools for predicting cytogenetic features of chromosomes on the basis of their protein composition and determined the genomic coordinates for all black bands of polytene chromosome 2R. By a PCNA immunostaining assay, we obtained the replication timetable for all the bands mapped. The results allowed us to compare replication timing between polytene chromosomes in salivary glands and chromosomes from cultured diploid cell lines and to observe a substantial similarity in the global replication patterns at the band resolution level. In both kinds of chromosomes, the intervals between black bands correspond to early replication initiation zones. Black bands are depleted of replication initiation events and are characterized by a gradient of replication timing; therefore, the time of replication completion correlates with the band length. The bands are characterized by low gene density, contain predominantly tissue-specific genes, and are represented by silent chromatin types in various tissues. The borders of black bands correspond well to the borders of topological domains as well as to the borders of the zones showing H3K27me3, SUUR, and LAMIN enrichment. In conclusion, the characteristic pattern of polytene chromosomes reflects partitioning of the Drosophila genome into two global types of domains with contrasting properties. This partitioning is conserved in different tissues and determines replication timing in Drosophila.

The Phylogenetic Origins and Evolutionary History of Holocentric Chromosomes

In eukaryotes, we can recognize two kinds of chromosomes, based on the location of the kinetochores. The majority of eukaryotes have monocentric chromosomes, in which kinetochoric activity is concentrated in a single locus. In several unrelated eukaryotic lineages, chromosomes are holocentric, having diffuse centromeric / kinetochoric activity along the length of the chromosome. Whether holocentric chromosomes are derived or ancestral is still under debate. This study uses the phylogenetic tree from Time Tree of Life project, comprising more than 50,000 sampled species, to reconstruct the evolution of holocentry. Asymmetrical two-state Markov (Mk2) models were compared with BiSSE models to assess sensitivity of our conclusions to possible effects of holocentry on lineage diversification rates. Our analyses based on Mk2 and BiSSE models inferred that the rate of transition from holocentric to monocentric chromosomes is two orders of magnitude higher than the reverse direction. The ancestral state of all eukaryotes is ambiguous depending on the model, inferred to be either monocentric (Mk2) or holocentric (BiSSE). Whatever the direction, the multiple transitions and high diversity of centromere organization across the tree of life are what we would expect if there are selective advantages to both chromosome types. Understanding those selective advantages is key to understanding how genetic information is organized and transmitted from one generation to the next, and why these major evolutionary transitions in centromere organization have occurred in the first place.

Response To Azacitidine Is Independent Of TP53 Mutations In Higher-Risk Myelodysplastic Syndromes (MDS) and Secondary Acute Myeloid Leukemia (sAML)

IntroductionHigher-risk myelodysplastic syndromes (MDS) progress to secondary acute myeloid leukemia (sAML) within months. Treatment with azacitidine (AZA) has been shown to prolong survival and delay progression to sAML. TP53 mutations are found in MDS with abnormal chromosome 5 or complex karyotype and are associated with poor prognosis. In particular, TP53 mutations confer resistance to lenalidomide in low-risk MDS with del(5q). However, whether TP53 mutations also influence response to AZA is unclear. Therefore, we analyzed the prevalence of TP53 mutations in a cohort of 100 patients with higher-risk MDS or sAML treated with AZA and correlated this to outcome. Patients and Methods100 cases of higher risk MDS (IPSS INT-2 or High, n= 53), MDS/MPN (n= 8) or sAML (≥ 20% marrow blasts, n= 39) were included who had received at least one complete cycle of AZA (75 mg/m2 d1-7 q28) and for whom a bone marrow biopsy was available before the start of treatment. Presence of TP53 mutations was determined by immunohistochemistry [Iwasaki et al, Pathol Int 2008; Jädersten et al, JCO 2011; Kulasekararaj et al, Br J Haematol 2013]. Staining was defined as positive if ≥5% of CD34+ cells showed strong nuclear staining. In addition, 37 randomly selected cases were confirmed by sensitive next-generation amplicon deep-sequencing, demonstrating good correlation between both methods. Response to AZA was determined using IWG2006 criteria. Kaplan-Meier curves were estimated from the first day of AZA treatment with the log-rank test to determine significance. Patients who subsequently underwent allogeneic transplantation were censored at that date. ResultsThirty-five patients (35%) had TP53 mutations. Both TP53 positive and negative cohorts were balanced in age (median 71 years) and number of administered AZA cycles (median 5/4, TP53 positive/negative, respectively). Of the TP53 positive patients, 68% had higher-risk MDS, 6% had MDS/MPN and 26% had sAML. Significantly more patients diagnosed with sAML had unmutated TP53 (46%, p=0.046). With regard to cytogenetics, only 23% of patients with TP53 mutations showed a good risk karyotype (KT) according to IPSS cytogenetic risk classification whereas 74% were diagnosed with poor risk KT. A closer look at the TP53 positive patients with poor risk cytogenetics revealed involvement of chromosome 5 aberrations in 77% compared to 33% in wild-type TP53 patients (p < 0.001). Of all 100 patients a total of 18 fulfilled the criteria for monosomal KT and 14 of these harbored TP53 mutations (77%, p < 0.001). There was no difference in pretreatment absolute neutrophil count or hemoglobin value in patients with or without TP53 mutations. The pretreatment median platelet count in TP53 mutated patients was significantly higher than in patients with wild-type TP53 (90 G/l vs. 43 G/l, respectively, p=0.021). The overall response rate (ORR) for patients with and without TP53 mutations treated with AZA was 74% (26/35) and 68% (44/65), respectively (n.s.). Quality of response did not show any differences between cohorts. However, hematologic improvement was significantly more frequent in patients with MDS and TP53 mutations than in MDS with wild-type TP53 (46% vs. 14%, p=0.015). Patients with complex KT and TP53 mutations responded as well as patients with wild-type TP53 to treatment with AZA (ORR 65% versus 53%, respectively, n.s.). Regarding the subgroup of patients with any kind of chromosome 5 abnormality within the complex KT group, there was a better ORR to AZA but no statistical significance between those with TP53 mutations compared to those with wild-type TP53 (75% vs. 43%, respectively). A similar result with respect to ORR was seen in the subgroup of patients with monosomal KT and the presence or absence TP53 mutations (ORR: 64% vs. 25%, respectively, n.s.). The median OS of the total cohort was 288 days, with a median OS of 212 days for TP53 positive patients and 353 days for TP53 negative patients (p = 0.731). ConclusionTP53 mutations in higher-risk MDS and sAML are frequent and predominantly associated with complex karyotype involving chromosome 5 abnormalities. In our cohort of 100 patients treated with AZA, the presence of TP53 mutations did not negatively impact on response to epigenetic therapy. However, although this analysis was not created for survival assessment it confirms the very poor survival for higher-risk MDS and sAML patients with TP53 mutations, in spite of response to AZA. Disclosures:Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Germing:Celgene: Honoraria, Research Funding. Platzbecker:Celgene Corp: Honoraria. Götze:Celgene Corp: Honoraria.

An analysis of complex chromosomal aberrations in seven cases of myelodysplastic syndromes by M-FISH and whole chromosome painting

Complex chromosomal aberrations (CCAs) can be detected in a substantial proportion of myelodysplastic syndrome (MDS). Comprehensive analysis of the chromosomal rearrangements in these CCAs has been hampered by the limitations of conventional cytogenetics (CC). Multiplex fluorescence in situ hybridization (M-FISH) is a new generation FISH technique which allows simultaneous identification of all the 24 human chromosomes. So it is very useful in clarifying CCAs, identifying cryptic interchromosomal rearrangements and characterizing marker chromosomes. But it also has some limitations. We used M-FISH and whole chromosome painting (WCP) to accurately refine the CCAs revealed by R-banding CC in seven cases with MDS. The composition and origin of 6 kinds of marker chromosomes, nine kinds of chromosomes with additional material undetermined and five kinds of derivative chromosomes undefined by CC were defined after M-FISH analysis. Four kinds of cryptic translocations overlooked by CC were found on derivative chromosomes and previously normal appearing chromosomes. In addition, M-FISH revealed some nonrandom aberrations which most frequently involved chromosome 17 (5/7) and -5/5q-(4/7). Fluorescence flaring is a main factor leading to misinterpretations. Some misclassified and missed chromosomal aberrations by M-FISH were corrected by WCP. M-FISH is a powerful molecular cytogenetic tool in clarification of CCAs. Complementary WCP can further identify misclassified and missed chromosomal aberrations by M-FISH. CC in combination with molecular cytogenetic techniques including M-FISH and WCP can more precisely unravel CCAs.

A GA-Based Document Clustering Method for Search Engines

In this paper, we present a novel genetic algorithm, called Multiple Search Genetic Algorithm (MSGA), for clustering the web pages returned by a search engine and providing a taxonomy of those web pages to the user. MSGA uses two different kinds of chromosomes (conservative and explorer) to improve the search capability as well as enhance the clustering result. The conservative chromosomes keep the better solutions found at each generation while the explorer chromosomes are used to increase the search directions to avoid falling into local minima. The proposed method can find the optimal solutions quickly via a multiple search strategy. Our simulation result shows that the proposed algorithm outperforms other algorithms. We also present a clustering search engine system, called Document Clustering Search Engine (DCSE). It is the DCSE that takes the responsibility for spawning agents for collecting the web pages from the meta-search engine and computing the similarity between the web pages. The user of the system will receive information that has been computed and sorted and web links that are ranked according to their relevance. The end result is that the amount of time required to filter out irrelevant information is highly reduced.

Whole Chromosome Painting and Multiplex Fluorescence In Situ Hybridization in Detecting Complex Chromosomal Aberrations in Myelodysplastic Syndromes.

Objective To explore the value of the technique of whole chromosome painting (WCP) and multiplex fluorescence in situ hybridization (M-FISH) in the detection of complex chromosomal aberrations (CCAs) of myelodysplastic syndromes (MDS).Methods M-FISH was used in seven MDS patients with CCAs detected by R-banding technique to refine CCAs, and to identify cryptic translocations and characterization of marker chromosomes. Dual-color WCP procedures were further performed in 7 cases to confirm some rearrangements detected by M-FISH.Results In all cases, M-FISH confirmed all results of R-banding.The composition and origin of 6 kinds of marker chromosomes, 9 kinds of chromosomes with additional material undetermined and 5 kinds of derivative chromosomes undefined by CC were defined after M-FISH analysis; 4 kinds of cryptic translocations overlooked by CC were found on derivative chromosomes and previously normal appearing chromosomes. In addition, M-FISH revealed some nonrandom aberrations: aberrations involving chromosome 17 (5/7) and -5/5q- (4/7) were the two most frequent aberrations. Fluorescence flaring is a main factor leading to misinterpretations. Some misclassified and missed chromosomal aberrations by M-FISH were corrected by WCP.Conclusions M-FISH is a powerful molecular cytogenetic tool in clarification of CCAs. Complementary WCP helped us identify misclassified and missed chromosomal aberrations by M-FISH. CC in combination with molecular cytogenetic techniques M-FISH and WCP can unravel complex chromosomal aberrations more precisely.

An effective coding approach for multiobjective integrated resource selection and operation sequences problem

In this paper, we consider an integrated Resource Selection and Operation Sequences (iRS/OS) problem in Intelligent Manufacturing System (IMS). Several kinds of objectives are taken into account, in which the makespan for orders should be minimized; workloads among machine tools should be balanced; the total transition times between machines in a local plant should also be minimized. To solve this multiobjective iRS/OS model, a new two vectors-based coding approach has been proposed to improve the efficiency by designing a chromosome containing two kinds of information, i.e., operation sequences and machine selection. Using such kind of chromosome, we adapt multistage operation-based Genetic Algorithm (moGA) to find the Pareto optimal solutions. Moreover a special technique called left-shift hillclimber has been used as one kind of local search to improve the efficiency of our algorithm. Finally, the experimental results of several iRS/OS problems indicate that our proposed approach can obtain best solutions. Further more comparing with previous approaches, moGA performs better for finding Pareto solutions.

MALE-BIASED MUTATION, SEX LINKAGE, AND THE RATE OF ADAPTIVE EVOLUTION

An interaction between sex-linked inheritance and sex-biased mutation rates may affect the rate of adaptive evolution. Males have much higher mutation rates than females in several vertebrate and plant taxa. When evolutionary rates are limited by the supply of favorable new mutations, then genes will evolve faster when located on sex chromosomes that spend more time in males. For mutations with additive effects, Y-linked genes evolve fastest, followed by Z-linked genes, autosomal genes, X-linked genes, and finally W-linked and cytoplasmic genes. This ordering can change when mutations show dominance. The predicted differences in substitution rates may be detectable at the molecular level. Male-biased mutation could cause adaptive changes to accumulate more readily on certain kinds of chromosomes and favor animals with Z-W sex determination to have rapidly evolving male sexual displays.

MALE-BIASED MUTATION, SEX LINKAGE, AND THE RATE OF ADAPTIVE EVOLUTION

An interaction between sex-linked inheritance and sex-biased mutation rates may affect the rate of adaptive evolution. Males have much higher mutation rates than females in several vertebrate and plant taxa. When evolutionary rates are limited by the supply of favorable new mutations, then genes will evolve faster when located on sex chromosomes that spend more time in males. For mutations with additive effects, Y-linked genes evolve fastest, followed by Z-linked genes, autosomal genes, X-linked genes, and finally W-linked and cytoplasmic genes. This ordering can change when mutations show dominance. The predicted differences in substitution rates may be detectable at the molecular level. Male-biased mutation could cause adaptive changes to accumulate more readily on certain kinds of chromosomes and favor animals with Z-W sex determination to have rapidly evolving male sexual displays.

New Features in Karyotype Structure and Origin of Saffron, Crocus sativus L.

Karyotype analyses of saffron, Crocus sativus L., was made using improved routine and C-banding techniques on metaphase plates in root tip cells of 2 accessions from Republic of Azerbaijan and Iran. In the routine technique fixative of Lewitsky and aceto-iron-hematoxylin stain were used. It was established that metaphase chromosomes can be arranged according to their size and morphological features in 8 triplets ranging in size from 11.58±0.13 μm (triplet 1) to 4.57±0.05 μm (triplet 8). Triplets 1 and 2 include subacrocentric, triplets 3, 4 and 8 metacentric and triplets 6 and 7 submetacentric chromosomes. Three chromosomes in each triplet, as a rule, are similar although in some triplets one of them is infrequently distinguishable from the 2 others. Triplet 5 shows an extreme difference so that it always contains 2 kinds of chromosomes: chromosome 5(1) and chromosomes 5(2, 3). Chromosome 5(1) is metacentric (r=1.40) and 6.04±0.13 μm in length, but chromosomes 5(2, 3) are subacrocentric (r=3.49) and noticeably smaller (5.41±0.09 μm). Application of the C-banding technique revealed heterochromatin segments: the sharpest, sharp and week. The sharpest heterochromatin was on telomere of the short arm of chromosome type (triplet) 3 and on proximal part of the long arm of chromosome type 1; a sharp heterochromatin on telomere of the short arms of chromosome types 1, 4 and on the satellites, and a weak heterochromatin on a centromeric region all of 24 chromosomes. Chromosome type 5(1) had a heterochromatin that was a little stronger on the centromeric and, considerably weaker on telomeric regions comparing to those of chromosome type 4. Accessions of saffron from Republic of Azerbaijan and Iran were not significantly distinguishable in karyotype structure. The species C. sativus in its places of cultivation is obviously a clone of one triploid plant originated spontaneously in the nature through crossing between 2 closely related species with participation of n and 2n gametes. Results from statistic analysis and appropriate photographic evidences are provided.

Occurrence of a Metacentric Macrochromosome B in Different Species of the Genus Astyanax (Pisces, Characidae, Tetragonopterinae).

Several cases of B chromosomes have been described for neotropical fish species. In some fish groups, this occurrence is sporadic, whereas in others it is more frequent. A high frequency of a metacentric macrochromosome B has been reported for Astyanax scabripinnis populations from different locations. In the present study, this kind of chromosome is reported for the first time in 2 other species of the same genus, A. fasciatus and A. schubarti. This coincidence, as well as the differential B chromosome frequency among the 3 Astyanax species, permit some considerations about its probable common origin and further distribution in this fish group.

Mitosis in eukaryotic cells: an overview of chromosome distribution.

The usual outcome of mitosis is 2 daughter cells with equal numbers and kinds of chromosomes (euploidy) rather than unequal numbers (aneuploidy). The more we learn about the underlying mechanisms the more remarkable this seems: the precise distribution of chromosomes depends critically upon structural disorder and instability, which would more commonly be associated with imprecision, that is, with aneuploidy. My aim is to review what is known about chromosome distribution in eukaryotes and to relate disorder and instability both to the normal outcome and to the origin of aneuploidy. The discussion is limited to orthodox mitosis and meiosis in organisms with localized kinetochores (on unorthodox mechanisms, see, e.g., Ref. 19; for broader reviews of mitosis, see Ref. 16 and 24).

Chromosome polymorphism in Japanese sika, Cervus (Sika) nippon.

Chromosome examination by blood culture was conducted on 35 Japanese sika, Cervus (Sika) nippon, revealing that the number of chromosomes varied according to individuals, such as 65, 66, 67 and 68. This numerical variation depended on the presence or absence of large metacentric chromosomes and submetacentric chromosomes in the autosomes. It was possible to observe six karyotypes according to the presence or absence of these two kinds of chromosomes. Polymorphism of these chromosomes is presumed to be formed when either large metacentric chromosomes or submetacentric chromosomes are formed following fusion of four acrocentric chromosomes in the autosomes at the centromere. The fundamental number was constant, 70 in all cases. This is considered to be due to the so-called Robertsonian translocation. The common phenomenon observed in all the individuals examined was the presence of one pair of middle-sized metacentric chromosomes in the autosomes. The X-chromosome was the largest among the acrocentric chromosomes and the Y-chromosome could be easily identified as a small submetacentric chromosome.

Positive and negative birefringence in chromosomes.

By using the optical properties of birefringence of DNA, the arrangement of these molecules has been studied in Dinoflagellate chromosomes and Dipteran polytene chromosomes. These latter are used, here, as a reference material. These observations have been made under a polarizing microscope on intact and stretched chromosomes.--Intact Dinoflagellate chromosomes show a positive birefringence, in contrast with polytene chromosomes bands which are negatively birefringent. From these observations one can deducd the preferential orientation of DNA filaments, in Dinoflagellates, normal to the chromosome axis, and in polytene chromosomes parallel to the same axis.--After stretching, these two kinds of chromosomes are negatively birefringent. In both cases, DNA molecules have been aligned along the stretch axis.--In Dinoflagellate chromosomes the passage from a positive to a negative birefringence is realized without any isotropic stage. The intermediary state presents a biaxial structure.

Cytogenetics of Mites and Ticks

There are three reviews of the cytogenetics of certain mites and ticks (43, 48, 75). The number of acarines studied cytologically more than doubled during the lO years between Sokolov's review and Oliver's first review. The number has now doubled again. Even with this increase in information our knowledge of the cytogenetics of acarines is still fragmentary. Over 30,000 species and 1700 genera have been de­ scribed and it is believed that a half million more species are extant (35). Neverthe­ less, data from our small cytogenetic sample indicate that an enormous amount of cytological and genetic diversity exists among mites and ticks. Somatic chromosome numbers range from 2 to 36, various sex chromosome systems are operative, and different kinds of chromosomes and reproductive mechanisms exist. The purpose of this review is to consider some of the fascinating aspects of the cytogenetics of mites and ticks by examining karyotypes, sex determination, and chromosome types.

Karyotypes and morphological variation in the sessile-flowered trilliums of North Carolina

Six populations of apparently morphologically similar sessile-flowered Trillium (known taxonomically as T. cuneatum Raf. and T. luteum (Muhl.) Harb.) observed growing at elevations between 800 and 1700 ft in the Appalachians of western North Carolina and eastern Tennessee exhibit a wide range of variability by observation. The extent of variability was assessed by using statistical and karyotypic methods of analysis and by intraspecific cross-pollination. A correlation exists between the mean coefficient of variability and (1) the size of the population and (2) the degree of geographic isolation. The number of types of each kind of chromosome (A–E), as resolved by cold treatment, varies by one type from population to population. The homozygosity for the six populations is 85.8%, and the distribution of euchromatin is not complex. Intraspecific cross-pollination was effective among 76% of the test crosses. A review of the statistics and other morphological data indicates that the presence or absence of anthocyanins, flower scent, and the degree of geographic isolation are the three major variants among the six populations. These kinds of Trillium grow sympatrically with T. grandiflorum (Michx.) Salisb., T. simile Gleason, T. erectum var. album (Michx.) Pursh, T. flexipes Raf. (T. gleasoni Fern.), and T. erectum L. The cumulative karyotype for the six stands indicates that these plants have not hybridized with any of the above species and varieties. Although these plants have been classified as T. cuneatum Raf. in some stands and as T. luteum (Muhl.) Harb. in others the data demonstrate that these six stands represent one discontinuous population of a sessile-flowered Trillium. Trillium discolor Wray, although relatively rare in western North Carolina, also grows here. Although T. discolor is not directly involved in this problem, its description and distinctive karyotype are included.

Diploid-Bud Formation in a Haploid &lt;i&gt;Oryza&lt;/i&gt; with Some Remarks on the Behaviour of Nucleolus in Mitosis

1. A fertile diplosporophyte derived from a haplosporophyte by bud-variation was separated as a clone from the mother stock. The root tip cells in the mother stock had 12 chromosomes and those in the fertile diplosporophyte had the doubled chromosome complement.2. There appeared several culms on which the panicles had fei tle grains mixed with some abortive flowers. Some adventitious roots from such culms consisted of haploid cells only, but others had completely diploid cells, and in other cases the root showed the constitution of a sectorial chimera of haploid and diploid tissue in longitudinal rows. A comparison in the sizes of the cells and the thickness of these roots was made and it was concluded that the sizes of the cells in respective tissue are firstly determined by the chromosome complements, but the propagation of the cells in the respective tissue is much influenced by other factors.The ratio of the cell volume of the haploid cells to that of the diploid cells is ca. 1:1.26 which is very near to 3√2, in other words the 2x-cells have about 2 times the volume of 1x-cells.3. As the cause of the bud variation in the present case an over-heating of the water of the culture pot in the sprouting period is suggested. The 2x-sporophytes propagate more rapidly and grow more vigorously than 1x-sporophyte and overhelm the latter. So we can easily see that in nature haplosporophytes produced in some perennial plant, may survive until when the 2x-bud variation appeared among them, an event which proved fatal to the 1x-plants.4. In early period of the mitosis, probably all the chromosomes are attached to the nucleolus either with their special points or with their end. The former are the naked or very thinly covered parts of the chromonemata, and not the surface of the hyalonema (the chromosome matrix). The nucleolar substance is probably concerned in the various stages of the chromosome cycle for all chromosomes, and not limited to particular ones such as the nucleolar or SAT-chromosomes. The latter kinds of chromosomes have longer naked or thinly covered chromonema neck, so that they are attached snore closely to the nucleolus.Here the writer wishes to express her thanks to Prof. K. Fujii for his kind advice in the course of this investigation. Her thanks are also due to the authorities of the Konosu Branch of the Imp. Agric. Exper. Station for the supply of the material.

An Abnormal Staining Capacity of the Sixth Salivary Gland Chromosome of a Strain of &lt;i&gt;Drosophila virilis&lt;/i&gt;

1. The sixth salivary gland chromosome of the New Orleans strain of D. virilis has a markedly lower staining capacity than that of other strains.2. This chromosome is also wider and longer than those in other strains. The arrangements of the bands, however, show no difference when crossed, a complete somatic synapsis occurs between this chromosome and the corresponding chromosome of any other strains in the heterozygous condition.3. Among 23 strains from various localities of Japan and China, as well as of America examined, none had a sixth chromosome like that of the New Orleans. Only a mutant stock, Gap2 hump/lethal 6a derived from the American Gap2 stock, contained such a chromosome.4. No difference was found in the frequency of crossing over between the sixth chromosomes of the New Orleans and that of other strains.5. In a new composite chromosome derived from such crossing over, the distal half from VI1A to VI1G is made up of the New Orleans chromosome and the other half of a section, VI1F to the proximal end, of a normal chromosome. This gives another case of cytological demonstration of the phenomenon of crossing over.6. A new allelomorph of Gap11 was discovered in the New Orleans stock.7. No significant difference in genetical behaviour exists between the New Orleans and normal strains. Nevertheless the markedly low viability and fertility of the stock with the recombination sixth chromosome is possibly due to some incompatibility between the two kinds of chromosomes.

Chromosome Studies in Tulipa and some related Genera.

The basic number of chromosomes in Tulipa is twelve and diploid, tetraploid, and hexaploid varieties and species occur. Great differences in size of chromosomes are found among the diploid species. The size of the nucleus is correlated with the size and number of the chromosomes it contains. Correlation of nuclear and somatic size only holds between tetraploid and diploid forms of the same species. Tulipa galatica Fresn. has sixteen chromosomes of which four are very small and unrepresented in the most nearly allied species. The increased number is regarded as being due to transverse fragmentation. Differences between the species exist in the form of the chromosomes and in their size relative to one another, even where no difference of number is present. Two pairs of chromosomes with sub-median attachment are found in Lilium, Fritillaria, and in most species of Tulipa. The doubtful correctness of the position of Calochortus in the Tulipeæ is emphasized by the cytological differences that distinguis it from the other genera. The chromosome numbers found are seven, nine, and ten. Well-marked satellites occur in this genus and in conjunction with differences in size and attachment enable each chromosome in the haploid set to be distinguished individually. Calochortus Vesta, a tetraploid series, has four representatives of each of the seven kinds of chromosomes. In the species (T. primulina, T. australis, T. Orphanidea) especially studied, synapsis involves the lateral pairing the leptotene stage of threads representing whole chromosomes. Later the split separating the conjugants reappears, and each conjugant becomes longitudinally split giving a tetrad consisting of four chromatids, which become separated in the first and second meiotic divisions. The forms take on by the tetrads in diakinesis -rings, crosses, etc.–are regarded as being due to the opening out alternately of the reductional and equational splits. No cytological proof of the existence of crossing over has been found. This work has been carried out whilst holding a studentship of the John Innes Horticultural Institution, and I have to record my gratitude to the late Dr. Bateson for his unfailing interest and encouragement.

The chromosomes in cross-fertilized Echinoid eggs

This paper deals with observations made on sections of cross-fertilized eggs of two kinds: (1) Toxopneustes eggs fertilized with Moira sperm, (2) Arbacia eggs fertilized with Moira sperm.The results of the study may be summarized as follows:1. The equatorial plate of the Moira-Toxopneustes cross shows a mixture of two kinds of chromosomes not sufficiently unlike one another to enable a positive distinction between the two.2. The equatorial plate of the Moira-Arbacia cross shows a mixture of two kinds of chromosomes, one variety long, the other variety short. These differences in form are correlated with the spermatozoon and the egg respectively.