The pattern of polytene chromosome conjugation and crossing-over in interspecific hybrids of Drosophila

Summary The pairing of polytene chromosomes was investigated in the hybrids between three closely related species of Drosophila belonging to the virilis species group. It was found that within the same hybrid different chromosome bands lost the ability to pair by differing degrees. Furthermore, the same chromosome sections paired with different frequencies depending on the hybrid involved. This study revealed that poor polytene chromosome pairing in the hybrids is not due to specific genetic interaction in the hybrids, but depends solely on the properties of the homologous loci themselves. It was also of interest to find whether the pattern of polytene chromosome somatic pairing resembled in some way the picture of chromosome synapsis during meiosis. To obtain evidence for this, crossing-over in the hybrid 5th chromosome was analyzed both genetically and cytologically (from salivary gland chromosome observations). It was found that the sections of the fifth chromosome which were characterized by a high frequency of conjugation in the salivary glands of hybrids also exhibited a high frequency of crossing-over in hybrid females. It may be concluded that sections of the polytene chromosome characterized by a low frequency of conjugation behave in the same manner in meiosis, and thus rarely take part in genetic recombination.     

Species-specific localization of DNA from pericentromeric heterochromatin on polytene chromosomes in the salivary gland cells and 3D-nuclear organization nurse cells in Drosophila virilis and Drosophila kanekoi (Diptera: Drosophilidae)

Microdissection of the chromocenter of D. virilis salivary gland polytene chromosomes has been carried out and the region-specific DNA library (DvirIII) has been obtained. FISH was used for DvirIII hybridization with salivary gland polytene chromosomes and ovarian nurse cells of D. virilis and D. kanekoi. Localization of DvirIII in the pericentromeric regions of chromosomes and in the telomeric region of chromosome 5 was observed in both species. Moreover, species specificity in the localization of DNA sequences of DvirIII in some chromosomal regions was detected. In order to study the three-dimensional organization of pericentromeric heterochromatin region of polytene chromosomes of ovarian nurse cells of D. virilis and D. kanekoi, 3S FISH DvirIII was performed with nurse cells of these species. As a result, species specificity in the distribution of DvirIII signals in the nuclear space was revealed. Namely, the signal was detected in the local chromocenter at one pole of the nucleus in D. virilis, while the signal from the telomeric region of chromosome 5 was detected on another pole. At the same time, DvirIII signals in D. kanekoi are localized in two separate areas in the nucleus: the first belongs to the pericentromeric region of chromosome 2 and another to pericentromeric regions of the remaining chromosomes.     

Localization of <Emphasis Type="Italic">Dras1</Emphasis> gene in polytene chromosomes of sibling species of <Emphasis Type="Italic">Drosophila virilis</Emphasis> group

The Dras1 gene was mapped by in situ hybridization to polytene chromosomes of several sibling species of the Drosophila virilis group and their hybrids. A 1037-bp fragment of Dras1 gene from the D. virilis genome was used as the probe. The gene sequence was localized in the region of a 25 A-B disk in chromosome 2 (in accordance with the D. virilis polytene chromosome map (Gubenko and Evgen’ev, 1984).     

The pattern of polytene chromosome synapsis in Drosophila species and interspecific hybrids

The pairing of polytene chromosomes was investigated in Drosophila melanogaster, Drosophila simulans and their hybrids as well as in species of the D. virilis group and in F₁ hybrids between the species of this group. The study of frequency and extent of asynapsis revealed non-random distribution along chromosome arms both in interspecific hybrids and pure Drosophila species. It is suggested that definite chromosome regions exhibiting high pairing frequency serve as initiation sites of synapsis in salivary gland chromosomes.     

Interspecies variability of number of bristles on dorsal surface of aedeagus in D. virilis species group and its genetic mapping with interspecies hybrids of D. virilis and D. lummei

The results of morphologic and hybrid analyses of the feature of the male reproductive system of sibling species in the virilis group were presented. Bristles appeared on the surfaces of male genitals (aedeagus). The occurrence of a specific expression of the examined feature in the phyllades of D. virilis group, the correspondence of both the number and distribution pattern of the bristles on surfaces of the aedeagus and developmental temperature in D. virilis and D. lummei, as well as the link between feature and sexual behavior, have been shown. Dominance of D. lummei phenotype in the interspecies D. virilis × D. lummei was found. The interspecies hybrids D. virilis and D. lummei were used for a genetic analysis of the variability of the examined feature. The significant influence of chromosomes 2 and 6 on the number of bristles on the aedeagus in hybrid males was shown. Furthermore, the correspondence between the effects of the autosomes 2 and 6 on the variability of the examined feature and the genetic status of the other chromosomes (the effect of interaction between genetic factors, chromosomes here) was revealed. The adaptive value of the examined feature related to the involvement in the formation of isolating barriers at the copulation stage is under discussion.     

Underreplication of satellite DNAs in polyploid ovarian tissue of Drosophila virilis

The satellite DNAs of Drosophila virilis have been examined in diploid and polyploid tissues by isopycnic ultracentrifugation and thermal denaturation experiments. Previous work has established that the satellite DNAs are under replicated in the polytene chromosomes of the salivary glands of D. virilis. The results of the present experiments demonstrate that this underreplication also takes place in the ovaries which contain nurse cells and follicle cells. These tissues are polyploid but do not show polytene chromosomes.     

Detection and location of three simple sequence DNAs in polytene chromosomes from virilis group species of Drosophila

In vitro synthesized RNAs complementary to the three satellite DNAs of Drosophila virilis have been used in a series of in situ hybridization experiments with polytene chromosomes from virilis group species. Gall and Atherton (1974) demonstrated that each of the satellites of D. virilis is comprised of many repeats of a distinct, seven base pair long, simple sequence. With few exceptions, copies of each of these simple sequences are detected in the chromocenters of all virilis group species. This is true even in species which do not possess satellite DNAs at buoyant densities corresponding to those of the satellite DNAs of D. virilis. Small quantities of the three simple sequences are also detected in euchromatic arms of several different species. The same euchromatic location may contain detectable copies of one, two, or all three simple sequence DNAs. The amounts of simple sequences at each location in the euchromatin may vary between species, between different stocks of the same species, and even between individuals of the same stock. The simple sequences located in the euchromatin appear to undergo DNA replication during formation of polytene chromosomes unlike those in heterochromatin. The locations of the euchromatic sequences are not the results of single chromosomal inversion events involving heterochromatic and euchromatic breakpoints.     

Localization of sequences coding for histone messenger RNA in the chromosomes of Drosophila melanogaster

In situ hybridization of sea urchin (Psammechinus miliaris, Lytechinus pictus and Strongylocentrotus purpuratus) histone messenger RNA has been used to map complementary sequences on polytene chromosomes from Drosophila melanogaster. The sea urchin RNA hybridizes to the polytene regions from 39D3 through 39E1-2, including both of these bands (39D2 may also be included). This region is identical to the one which hybridizes most heavily with non-polyadenylated cytoplasmic RNA from D. melanogaster tissues. Sea urchin mRNAs coding for several individual histones each hybridize across the entire region from 39D3 (or D2) through 39E1-2, as would be expected if the individual mRNA sequences are interspersed. In view of the apparently even distribution of sequences complementary to histone mRNA within the 39D3-39E1-2 region, the significance of the several polytene bands in this region remains an open question. Biochemical characterization of the hybrids between sea urchin histone mRNA and D. melanogaster DNA suggests that sea urchin mRNAs for several of the histone classes have some portions which retain enough sequence homology with the D. melanogaster sequences to form hybrids, although the hybrids have base pair mismatches. In situ hybridization of chromosomes in which region 39D-E is ectopically paired show no evidence of sequence homology in the chromosome region with which 39D-E is associated.     

Transposition of mobile genetic elements in interspecific hybrids of Drosophila

In situ hybridization of labeled DNA of four mobile dispersed genetic elements (mdg), isolated from D. melanogaster and C. virilis genomes, with polytene chromosomes of the larvae of several Drosophila species has been carried out. The data show that the mdg elements exhibit a high degree of species specificity. The same conclusions are derived from filter hybridization using ³²P-labeled D. melanogaster and D. virilis DNA and cloned mdg sequences immobilized on nitrocellulose filters. We attempted to induce transpositions (jumping) of mdg elements specific for D. virilis chromosomes to the chromosomes of related species (e.g. D. littoralis Meigen) originally lacking the representatives of this family of repeats. For this purpose we produced hybrid stocks with synthetic karyotoypes characterized by different combinations of D. virilis homologous chromosomes and hybrid chromosomes. In one of such stocks we did find by in situ hybridization the insertion of a D. virilis mdg element into the fifth chromosome of D. littoralis Meigen. The transposition (jumping) took place in the only region where somatic pairing between the fifth chromosomes of D. virilis and D. littoralis occurs more or less regularly in the hybrids. Since crossing-over in hybrid chromosomes of males is excluded in such synthetic stocks, gene conversion may be responsible for this transposition. The possible bearing of the phenomenon observed on the problem of hybrid dysgenesis is discussed.     

Genetics of esterases in Drosophila. III. Influence of different chromosomes on esterase pattern in Drosophila

The present report presents the results of starch and polyacrylamide gel electrophoretic studies of the influence of the X chromosome on the expression of esterase-6 in D. melanogaster × D. simulans hybrids heterozygous for locus Est-6 as well as studies of the influence of autosomes on esterase expression in Drosophila of the virilis group. A differential expression of esterase-6 has been detected in D. melanogaster × D. simulans hybrid males. A differential decrease in the activity of esterase-6 (both F and S allozymes) derived from D. melanogaster has been noted. In hybrid females, the activity of parental esterases is the same. It is suggested that the X chromosome regulates the expression of esterase-6 in D. melanogaster. Analysis of individuals obtained in different schemes of crosses between different species of Drosophila of the virilis group by use of stocks marked with mutations in various chromosomes indicates that other autosomes (in particular, autosomes 4 and 5) also influence the phenotypic expression of esterases (which are controlled by genes located on the second chromosome).     

Cytofluorometry and visualisation of DNA base composition in the chromosomes of Drosophila melanogaster

The DNA base composition determined cytofluorometrically with the dyes CMA and DAPI in individual mitotic chromosomes of Drosophila melanogaster agrees very well with reference data obtained by hybridisation. Measurements in polytene chromosomes showed: (1) The base composition in the chromocenter, in chromosome 4 and bands X 1 and 3R 81 is lower than would be expected if they consisted of satellite DNAs only. (2) In the chromosome arms, bands with deviating base composition were found also where no satellite DNAs have been localized. With two visualisation methods — a photographic technique and image analysis — a complex pattern of base composition heterogeneity in the arms of the polytene chromosomes was established. In part this pattern may reflect the intercalary heterochromatin shown by weak point behaviour, ectopic pairing, and late replication.     

COMPARTMENTALIZATION OF THE DEVELOPING MACRONUCLEUS FOLLOWING CONJUGATION IN STYLONYCHIA AND EUPLOTES

The development of the macronucleus following conjugation in the hypotrichous ciliates Euplotes and Stylonychia has been examined with the electron microscope. Banded polytene chromosomes can be seen in thin sections of the macronuclear anlagen during the early periods of exconjugant development. As the chromosomes reach their maximum state of polyteny, sheets of fibrous material appear between the chromosomes and transect the chromosomes in the interband regions. Individual bands of the polytene chromosomes thus appear to be isolated in separate compartments. Subsequently, during the stage when the bulk of the polytenic DNA is degraded (1), these compartments swell, resulting in a nucleus packed with thousands of separate spherical chambers. Individual chromosomes are no longer discernible. The anlagen retain this compartmentalized condition for several hours, at the end of which time aggregates of dense material form within many of the compartments. The partitioning layers disperse shortly before replication bands appear within the elongating anlagen, initiating the second period of DNA synthesis characteristic of macronuclear development in these hypotrichs. The evidence presented here suggests that the "chromatin granules" seen in the mature vegetative macronucleus represent the material of single bands of the polytene chromosomes seen during the earlier stages of macronuclear development. The possibility is also discussed that the degradation of DNA in the polytene chromosomes may be genetically selective, which would result in a somatic macronucleus with a different genetic constitution than that of the micronucleus from which it was derived.     

Dominance status of shape of male genitalia in interspecific crosses of some Drosophila virilis group species

The heritability of the shape of the main species-specific morphological trait for the Drosophila virilis group-the male mating organ has been analyzed using the hybrid males D. virilis × D. lummei and D. virilis × D. novamexicana. The results suggest an increase in the share of the characters with a recessive status in the evolutionarily younger species and demonstrate the role of sex chromosomes in the implementation of a dominant or recessive status of the trait. The roles of additive and epistatic components of the total variation in the evolution of the dominance status, shown in several known theoretical models and confirmed by our data, are considered. The published data on sterility of hybrid males in interspecific crosses are discussed from the standpoint of the evolution of dominance.     

Transposition of the hobo element in Drosophila melanogaster somatic cells

Somatic mutation and recombination test on wing cells of Drosophila melanogaster showed that the recombination frequency in the somatic tissues of strains studied correlated with the presence of a full-length copy of the hobo transposable element in the genome. Transposition of hobo in somatic tissue cells at a frequency 3.5 × 10^?2 per site per X chromosome was shown by fluorescence in situ hybridization with salivary gland polytene chromosomes of larvae of one of the D. melanogaster strains having a full-length hobo copy.     

Cytogenetics of the trigeneric hybrid, (Hordeum vulgare ×Triticum aestivum) ×Secale cereale

Summary Chromosome pairing was studied in hybrids of (Hordeum vulgare ×Triticum aestivum) ×Secale cereale. Chiasma frequency per cell varied from 1.94 to 3.16 between the different hybrids. This variation was attributed to genetic variability in rye parents which affected homoeologous pairing. The pairing of rye chromosomes as revealed by Giemsa C-banding was a combination of nonhomologous association between rye chromosomes and associations with chromosomes of wheat and barley. Contribution No. 634 Ottawa Research Station     

Genomic relationships between hexaploid Helianthus resinosus and diploid Helianthus annuus (Asteraceae)

Genus Helianthus comprises diploid and polyploid species. An autoallopolyploid origin has been proposed for hexaploid species but the genomic relationships remain unclear. Mitotic and meiotic studies in annual Helianthus annuus (2n = 2x = 34) and perennial Helianthus resinosus (2n = 6x = 102) as well as the F₁ hybrids between both species were carried out. Chromosome counting confirmed the hybrid origin of the latter plants and their tetraploid condition. Bivalents in hybrids ranged from 12 to 28 ( $ \bar{x} $  = 20.8). Univalents, trivalents and quadrivalents were also observed. Meiotic products comprised dyads, triads and normal tetrads and pollen grains were heterogeneous in size. These observations suggest the occurrence of 2n pollen in addition to the expected n. Genomic in situ hybridization (GISH) of total H. annuus DNA on H. resinosus chromosomes rendered weak but uniform signals; similar hybridization pattern was observed using three other annual species. Hybridization with H. annuus probe performed on root tip cells of F₁ H. annuus × H. resinosus hybrids revealed 17 chromosomes with a strong hybridization signal. GISH in hybrid meiocytes distinguished chromosomes from parental species and revealed autosyndetic pairing of H. resinosus chromosomes, allosyndetic pairing in bivalents, trivalents and quadrivalents, and the presence of univalents derived from parents, H. annuus and H. resinosus. Results obtained from classical and molecular cytogenetics do not support H. annuus as a direct ancestor of H. resinosus. The occurrence of allosyndetic pairing and the relatively high fertility of the F₁ hybrids point to the possibility that useful genes could be transferred from H. resinosus to cultivate sunflower, although the effective rate of recombination has not been evaluated. GISH method proved effective to recognize parental chromosomes in H. annuus × H. resinosus progeny.