Absence of ribosomal DNA amplification in the meroistic (telotrophic) ovary of the large milkweed bug Oncopeltus fasciatus (Dallas) (Hemiptera: Lygaeidae)

Empty-spiracles is maternally expressed and essential for neurodevelopment and early embryo determination in Rhodnius prolixus

Since empty-spiracles (ems) was identified and characterized in Drosophila melanogaster as a head-gap gene, several studies have been carried out in other insect orders to confirm its evolutionary conserved function. Using the blood-sucking bug Rhodnius prolixus as biological model, we found an ems transcript with three highly conserved regions: Box-A, Box-B, and the homeodomain. R. prolixus embryos silenced by parental RNAi for two of these ems conserved regions showed both maternal and zygotic defects. Rp-emsB fragment results in early lethal embryogenesis, with eggs without any embryonic structure inside. Rp-emsB expression pattern is only maternally expressed and localized in the ovary tropharium, follicular cells, and in the unfertilized female pronucleus. Rp-emsA fragment is zygotically expressed during early blastoderm formation until late developmental stages in two main patterns: anterior in the antennal segment, and in a segmentary in the neuroblast and tracheal pits. R. prolixus knockdown embryos for Rp-emsA showed an incomplete larval hatching, reduced heads, and severe neuromotor defects. Furthermore, in situ hybridization revealed a spatial and temporal expression pattern that highly correlates with Rp-ems observed function. Here,Rp-ems function in R. prolixus development was validated, showing that empty-spiracles does not act as a true head-gap gene, but it is necessary for proper head development and crucial for early embryo determination and neurodevelopment.

New data on karyotype, spermatogenesis and ovarian trophocyte ploidy in three aquatic bug species of the families Naucoridae, Notonectidae, and Belostomatidae (Nepomorpha, Heteroptera)

We report the karyotype, some aspects of spermatogenesis, and ovarian trophocytes ploidy in three aquatic bug species: Ilyocoris cimicoides (Linnaeus, 1758), Notonecta glauca Linnaeus, 1758, and Diplonychus rusticus Fabricius, 1871 from previously unexplored regions - South Europe (Bulgaria) and Southeast Asia (Vietnam). Our results add considerable support for the published karyotype data for these species. In I. cimicoides, we observed achiasmate male meiosis - the first report of achiasmy for the family Naucoridae. More comprehensive cytogenetic studies in other species of the Naucoridae are required to elucidate the role of achiasmy as a character in the systematics of the family. Our observations on the association between phases of spermatogenesis and developmental stages in I. cimicoides and N. glauca differ from the previously published data. In these species, we assume that the spermatogenesis phases are not strongly associated with certain developmental stages. For further cytogenetic studies (on the Balkan Peninsula), we recommend July as the most appropriate month for collection of I. cimicoides and N. glauca. In the ovaries of both species, we studied the level of ploidy in metaphase and interphase trophocytes. In I. cimicoides, diploid and tetraploid metaphase trophocytes were found. Heteropycnotic elements, observed in interphase trophocytes of this species, represented the X chromosomes. It allowed us to determine the trophocytes ploidy at interphase (2n was repeated up to 16 times). The situation with N. glauca was different. The metaphase trophocytes were diploid and we were not able to determine the ploidy of interphase trophocytes since such conspicuous heteropycnotic elements were not found. The scarce data available suggest a tendency for a low level of trophocyte ploidy in the basal infraorders (Nepomorpha and Gerromorpha) and for a high level in the more advanced Pentatomomorpha. Data about this character in species from other infraorders are needed to confirm that tendency.

Nucleoprotein cytochemistry during oogenesis in a unisexual fish, Poecilia formosa

Cytochemical methods and electron microscopy were used to study changes in the chemical composition of nuclear, nucleolar and perinuclear bodies during the early stages of oocyte development in Poecilia formosa, an apomictic species of fish that produces only female offspring. Prominent accumulations of ribonucleoprotein (RNP) occur in nucleoli and appear on either side of the nuclear envelope during diplotene. In certain planes of section, RNP material seems to be in transit across this interface. En bloc acid extractions or RNAase treatment abolished basophilia and markedly reduced the electron density of both nucleoli and cytoplasmic nucleolar-like bodies. DNA-specific fluorescent probes such as mithramycin failed to reveal nucleolar cores in poeciliid oocytes, although the same procedures showed unequivocal localization of GC-rich DNA cores within multiple nucleoli of diplotene oocytes from Xenopus laevis or the rainbow trout, Salmo gairdneri. Also, cytological hybridization studies, utilizing [3H]rRNA as a probe for nucleolar DNA, failed to show localizations of silver grains above nucleoli in autoradiograms of poeciliid oocytes. Feulgen-stained pachytene oocytes of P. formosa have twice the number of chromosome strands seen in similar stages of oocytes from two, related bisexual species, P. mexicana and P. latipinna. Although the bivalent nature of these chromosomes could not be resolved with the light microscope, configurations resembling, but not identical to, synaptonemal complexes were identified by electron microscopy.

Development and cellular differentiation of an insect telotrophic ovary (Rhodnius prolixus)

Differentiation events accompanying the larval-adult ovarian transformation in Rhodnius prolixus can be divided into three phases: proliferative phase (unfed to 3 days post-feed or DPF), early differentiation phase (9-15 DPF) and late differentiation phase (16 DPF to moult at 21 DPF). Ovarioles remain morphologically larval until feeding initiates development. The unfed ovariole contains germ cells surrounding a central trophic core region with the 'germarial lumen' occupying the basal region of the tropharium immediately above the pre-follicular tissue. Mitosis of germ cells during the proliferative phase results in a progressive increase in tropharial size with no differentiation of tissues. Regional specialization within the ovariole marks the beginning of the early differentiation phase. A zone of oocytes is established at the base of the tropharium with nuclei containing synaptonemal complexes and condensing chromosomes. Nurse cell differentiation is characterized by nucleolar elaboration and nucleo-cytoplasmic transport, the cytoplasm becoming rich in ribosomes. Autoradiographic results suggest that functional nurse cell-oocyte divergence occurs concurrently with morphological divergence. Pre-follicular tissue is divided into apical and basal zones with apical zone differentiation occurring during early and late differentiation phases.

Frequencies of circular units of nucleolar DNA in oocytes of two insects, Acheta domesticus and dytiscus marginalis, and changes of nucleolar morphology during oogenesis

The organization of the extrachromosomal nucleolar material in oocytes of two insect species with different ovary types, the house cricket Acheta domesticus (panoistic ovary) and the water beetle Dytiscus marginalis (meroistic ovary), was studied with light and electron microscopic techniques. Stages early in oogenesis were compared with fully vitellogenic stages (mid-to-late diplotene). The arangement of the nucleolar material undergoes a marked change from a densely aggregated to a dispersed state. The latter was characterized by high transcriptional activity. In spread and positively stained preparations of isolated nucleolar material, a high frequency of small circular units of transcribed rDNA was observed and rings with small numbers (1--5) of pre-rRNA genes were predominant. The observations suggest that the 'extra DNA body' observed in early oogenic stages of both species represents a dense aggregate ofnumerous short circular units of nucleolar chromatin, with morphological subcomponents identifiable in ultrathin sections. These apparently remain in close association with the chromosomal nucleolar organizer(s). The observations further indicate that the individual small nucleolar subunit circles dissociate and are dispersed as actively transcribed rDNA units later in diplotene. The results are discussed in relation to principles of the ultrastructural organization of nucleoli in other cell types as well as in relation to possible mechanisms of gene amplification.

Ribosomal DNA and its expression in Drosophila melanogaster during growth and development

Ribosomal DNA content has been determined in several adult and larval tissues of Drosophila melanogaster. Underreplication of rRNA genes was observed in polytenic salivary glands of larvae. On the contrary, polytenic/polyploid ovaries showed no decrease in rDNA. It is concluded that polyteny is not necessarily associated with underreplication of rDNA. No other tissue examined displayed any change in rDNA redundancy. Third-instar-larvae showed a decrease in rDNA amount which might be partly accounted for by underreplication of rDNA in salivary glands. No such decrease was seen in pupae. Bobbed genotypes were essentially similar to wild type in all tissues except salivary glands. In this case, it was found that the extent of underreplication is less in bobbed as compared to wild genotypes. Ribosomal DNA activity was examined in various tissues of Drosophila melanogaster. The rates of rRNA synthesis vary greatly between various tissues. It is concluded that a control at the level of gene activity operates as differences in the amount of precursor rRNA synthesized can be observed both in flies of varying rDNA contents as well as in various tissues of the same genotype.