Genetic heterogeneity and accelerated responses to directional selection in Drosophila

Summary Strains set up from single inseminated founder females of D. melanogaster from the same population have been previously shown to differ genetically for the incidence of flies with more than four scutellar chaetae (additional chaetae). Therefore the base population is polymorphic for this trait. This was exploited by carrying out directional selection in lines derived from those strains initially having a high mean chaeta number. This led to far more rapid responses to selection than were obtained in lines derived from strains with lower means, such that in one line a continuous accelerated response was observed for the first 12 generations of selection. A correlated response was found for sternopleural chaeta number at generation 19 of selection, showing that at least some genes may affect both traits.     

A pair of closely linked genes controlling high scutellar chaeta number in Drosophila

Summary 1. A line selected for high scutellar chaeta number reached a mean of about 16 chaetae in females and 13.5 in males at the 69th generation of selection following an accelerated response to selection which commenced at generation 65 and added five chaetae. 2. The accelerated response can probably be explained in terms of two recessive high chaeta number genes 1.05 cM apart, and which are located between po and vg on chromosome II. The gene closest to vg was found to be scabrous, sca, which causes rough eyes when homozygous and has a pleiotropic effect on scutellar chaeta number. The gene was found in one of the strains used in setting up the selection lines. 3. The results are discussed in relation to other theories of control of the scutellar chaeta system.     

Review of Chinese Oligaphorurini (Collembola, Onychiuridae) with descriptions of two new Palaearctic species

A checklist of Chinese Oligaphorurini is given. Two new Chinese species, Micraphorura changbaiensissp. n. and Oligaphorura pseudomontanasp. n., are described from Changbai Mountain Range. Micraphorura changbaiensis sp. n. has the same dorsal pseudocelli formula and number of papillae in Ant. III sensory organ as Micraphorura uralica, but they can be easily distinguished by number of chaetae in Ant. III sensory organ, ventral pseudocelli formula, ventral parapseudocelli formula, number of pseudocelli on subcoxa 1 of legs I-III, dorsal axial chaeta on Abd. V and number of chaetae on tibiotarsi. Oligaphorura pseudomontana sp. n. is very similar to the species Oligaphorura montana having an increased number of pseudocelli on body dorsally, well marked base of antenna with 1 pseudocellus and 3 pseudocelli outside, subcoxa 1 of legs I-III with 1 pseudocellus each, dorsally S-chaetae formula as 11/011/22211 from head to Abd. V, S-microchaeta present on Th. II-III, claw without inner teeth and with 1+1 lateral teeth, and unguiculus with basal lamella; but they can be separated easily by the number of pseudocelli on Abd. V and VI terga, parapseudocelli on the body, number of chaetae on Th. I tergum, and number of chaetae on tibiotarsi. A key to Chinese species of Oligaphorurini is provided in the present paper.     

Endonura Cassagnau in Iran,with a key to species of the genus (Collembola,Neanuridae, Neanurinae)

Three new species of Endonura are described from Iran. Endonura dichaeta sp. n. can be recognized by an ogival labrum, head without chaetae O and E, chaeta D connected with tubercle Cl, tubercle Dl with five chaetae on head, absence of tubercles Di on thorax I and tubercle (Di+Di) of thorax V with 2+2 chaetae. Endonura ceratolabralis sp. n. is characterized by large body size, reduction of labral chaetotaxy, ogival labrum, head without chaeta O and fusion of tubercles Di and De on first thoracic segment. Endonura persica sp. n. is distinguished from its congeners by a nonogival labrum, absence of chaeta O, tubercles Dl and (L+So) with five and eight chaetae respectively and claw with inner tooth. The key to all species of the genus is given.     

Chaetae and chaetogenesis in polychaetes (Annelida)

Annelid chaetae are epidermal extracellular structures that are in general clearly visible from the exterior. Their structure is highly diverse, especially within the Polychaeta, and each species shows a specific pattern of chaetae. Chaetae have therefore gained immense significance for species determination, making them the best studied structures in polychaetes. The shape of chaetae is determined by the temporal and spatial modification of the microvilli pattern of a single cell, the chaetoblast. As chaetae are species specific, the process of their formation must be under strict control and the information needed to form certain chaetae must be highly conservative. It can be assumed that corresponding chaetogenesis is caused by commonly inherited information. Thus, comparative chaetogenesis can help to test hypotheses on the homology of certain types of chaetae and help to unravel the influence of functional constraints on the shape of chaetae. Different types of chaetae are compared here and the present state of our knowledge of their structure and formation is used to present some homology hypotheses. There are some strong arguments for a homology of uncini and certain hooks and hooded hooks. Acicula are compared to other supportive setae and the significance of the arrangement of chaeta for phylogenetic considerations is shown. Coding issues are provided in order to facilitate inclusion of information on chaetae into data matrices.     

Chaetal arrangement and chaetogenesis of hooded hooks in Lumbrineris (Scoletoma) fragilis and Lumbrineris tetraura (Eunicida,Annelida)

As the structure and arrangement of chaetae are highly specific for annelid species and higher taxonomic entities, we assume that rather conservative information guarantees formation of specific chaetae. Each chaeta of an annelid is formed within an ectodermal invagination, and the modulation of the apical microvilli pattern of the basalmost cell of this invagination determines the structure of the chaeta. Any hypothesis of the homology of chaetae could thus be tested by examining the process of chaetal formation. Investigations into the ultrastructure and formation of hooded hooks in different capitellids and spionids revealed that these chaetae can be homologized. The hood of each of their hooded hooks is formed by elongation of two rings of microvilli peripheral to the chaetal anlage, which give rise to the inner and outer layers of the hood. The hood layers are well separated and surround an empty space. Superficially similar hooded hooks are described for certain Eunicida. Presently available cladistic analyses suggest that the hooded hooks of eunicidans evolved independently of those in Capitellidae and Spionidae. Compared with the latter two families, we therefore expected to find differences in chaetogenesis of the hooded hooks in the eunicids Lumbrineris (Scoletoma) fragilis and Lumbrineris tetraura (Lumbrineridae). This was the case. In these eunicidans, the hood was formed by the bisected apical wall of the chaetoblast right after the mid‐apical section of the chaeta had been sunk deeply into the chaetoblast during its formation. The apical wall generated a brush of microvilli that preformed the hood. Because the microvilli of the hood showed some accelerated differentiation, they soon merged with those of the slowly growing setal shaft to form the broad manubrium of the hooded hook in lumbrinerids. Our study confirms the predicted differences in chaetogenesis of the superficially similar hooded hooks of capitellids and spionids compared with those of eunicids.     

Broadening the definition of the genus Thalassaphorura Bagnall, 1949 (Collembola,Onychiuridae) with a new aberrant species from China

A new species belonging to the tribe Thalassaphorurini, Thalassaphorura problematica sp. n., is described from Northeast China. The new species is closest to the large genus Thalassaphorura by its simple vesicles in PAO and its furcal rudiment, but it does not fit the definition of the genus by the absence of chaeta d0 on head, the number of chaetae in the distal whorl of tibiotarsi and the labium type. We discuss the relative weakness of these last characters at generic level, which lead us to assign the new species to Thalassaphorura instead of erecting a new genus. The diagnosis of Thalassaphorura is broadened accordingly.     

Locomotion and fine structure of parapodia in <Emphasis Type="Italic">Myzostoma cirriferum</Emphasis> (Myzostomida)

Most myzostomids are ectocommensals of crinoids on which they move freely. Their locomotion is ensured by five pairs of parapodia located laterally below their trunk. Each parapodium in Myzostoma cirriferum is a conical structure that includes a hook-like chaeta, replacement chaetae and an aciculum. Structure and ultrastructure of the myzostomid chaetae are similar to those of polychaetes: they are formed by a chaetoblast, which gives rise to microvilli where chaetal material is assembled on the outer surface. Myzostoma cirriferum walks on its host. It moves the anterior part, the posterior part or the lateral parts forwards but is able to rotate of 180° on itself. Its locomotion entirely depends on parapodial motions and not on trunk movements. Three pairs of muscles are involved in parapodial motions: parapodium flexor and parapodium extensor, aciculum protractor and aciculum retractor, and hook protractor with conjunctor. A functional model is proposed for explaining the global motion of a parapodium in M. cirriferum that may be extended to all ectocommensal myzostomids.     

Fridericia larix sp. nov. (Enchytraeidae, Oligochaeta) from Irish soils

A new species of Fridericia (Enchytraeidae, Oligochaeta) is described from soils in Ireland. It was found during sampling campaigns in the framework of a comprehensive taxonomic revision of the genus (Schmelz, 2003. Taxonomy of Fridericia (Oligochaeta, Enchytraeidae). Revision of species with morphological and biochemical methods. Abh. Naturw. Ver. Hamburg, N.F. 38, 1–415, figs. 1–73), but it was not included in that study. Fridericia larix sp. nov., named in reference to the type locality, belongs to the large and taxonomically difficult group of species with two diverticula per spermatheca. It is distinguished from all known congeners by the following combination of characters: (1) a maximum of four chaetae in ventral preclitellar bundles; (2) oesophageal appendages poorly branched; (3) no pharyngeal glands in segment VII; (4) coelomocytes without refractile vesicles; (5) clitellum girdle-shaped, cell distribution alike on all sides; (6) bursal slit of male copulatory organ mainly transverse; (7) no subneural glands; (8) spermathecal diverticula not stalked. Further distinguishing characters are: (9) an asymmetrical arrangement of chaetae in the first lateral postclitellar bundles, with one large chaeta and one small chaeta per bundle (Fig. 1b, “lc XVI”); (10) the length ratio of spermatozoa to spermatozoal nuclei (6:1–7:1); and (11) a wavy inner surface in parts of the epithelium of the spermathecal ampulla (Fig. 1i, arrow).See also Electronic Supplement at: http://www.senckenberg.de/odes/05-05.htm     

Getting to the root of fireworms' stinging chaetae—chaetal arrangement and ultrastructure of Eurythoe complanata (Pallas, 1766) (Amphinomida)

Amphinomid species are since long known to cause urtication upon contact with the human skin. Since it has been reported that amphinomid chaetae are hollow, it has repeatedly been suggested that poison is injected upon epidermal contact. To test predictions for the structural correlate of such a stinging device we studied the structure and formation of chaetae in the fireworm Eurythoe complanata (Amphinomida). Neither the structure of the chaetae nor their formation and their position within the parapodium provide evidence for their function as hollow needles to inject poison. The chaetae even turned out to be not hollow, but containing calcareous depositions. The latter most likely cause artificial ruptures of delicate chitin lamellae in the inner of the chaeta when treated with acidic fixatives. Inorganic calcium compounds harden the chaetae and make them brittle so that they break easily. Additional information on the structure of the chaetal sac, the site of formation and the acicula do not contradict the position of the Amphinomida within Annelida as revealed by phylogenomic studies.     

Effects of extreme temperatures on phenotypic variation and developmental stability in Drosophila melanogaster and Drosophila buzzatii

The effect of stressful and nonstressful rearing temperatures on phenotypic variation of four quantitative characters (thorax length, wing length, number of sternopleural chaetae, number of arista branches) and on developmental stability (fluctuating asymmetry) of the three latter characters was estimated in two Drosophila species: Drosophila melanogaster and Drosophila buzzatii . In both species, a general trend for increasing of phenotypic variation and fluctuating asymmetry at stress temperatures was observed; in fluctuating asymmetry, this effect was more pronounced. An increase of phenotypic variation under stress was shown for all characters examined except sternopleural chaeta number in D. buzzatii . Comparison of species responses suggests that the increase of variation in D. melanogaster was somewhat higher than in D. buzzatii .     

Genetic heterogeneity among the founders of laboratory populations of Drosophila IV. Scutellar chaetae in different environments

The incidence of flies with more than four seutellar chaetae (additional chaetae) has been followed for up to 3 years in 4 lines at 15°C, 20°C, 25°C and 27.5°C derived from each of 3 strains, initially set up from single inseminated females of Drosophila melanogaster, collected in the wild single the same locality at the same time. Within each set of temperature lines, the incidence of additional chaetae was highest in one strain, intermediate in another strain, and lowest in the third. This shows that the differences between lines can be at least partly attributed to differences between the founder females of the three strains, although some divergence occurred between lines within strains in some cases. With respect to quantitative traits, the results indicate that in situations where new habitats are occupied by a small number of colonists, the nature of the founder female is possibly of greater importance than the different environments of the new habitats.Some of this work was supported by the Australian Research Grants Committee.     

Homology and Evolution of the Chaetae in Echiura (Annelida)

Echiura is traditionally regarded as a small phylum of unsegmented spiralian worms. Molecular analyses, however, provide unquestionable evidence that Echiura are derived annelids that lost segmentation. Like annelids, echiurans possess chaetae, a single ventral pair in all species and one or two additional caudal hemi-circles of chaetae in two subgroups, but their evolutionary origin and affiliation to annelid chaetae are unresolved. Since annelids possess segmental pairs of dorsal (notopodial) and ventral (neuropodial) chaetae that are arranged in a row, the ventral chaetae in Echiura either represent a single or a paired neuropodial group of chaetae, while the caudal circle may represent fused rows of chaetae. In annelids, chaetogenesis is generally restricted to the ventral part of the notopodial chaetal sac and to the dorsal part of the neuropodial chaetal sac. We used the exact position of the chaetal formation site in the echiuran species, Thalassema thalassemum (Pallas, 1766) and Echiurus echiurus (Pallas, 1767), to test different hypotheses of the evolution of echiurid chaetae. As in annelids, a single chaetoblast is responsible for chaetogenesis in both species. Each chaeta of the ventral pair arises from its own chaetal sac and possesses a lateral formation site, evidencing that the pair of ventral chaetae in Echiura is homologous to a pair of neuropodia that fused on the ventral side, while the notopodia were reduced. Both caudal hemi-circles of chaetae in Echiurus echiurus are composed of several individual chaetal sacs, each with its own formative site. This finding argues against a homology of these hemi-circles of chaetae and annelids’ rows of chaetae and leads to the hypothesis that the caudal chaetal rings evolved once within the Echiura by multiplication of ventral chaetae.     

Two new species of Spiochaetopterus (Annelida: Polychaeta) from Sagami Bay and Tokyo Bay, Central Japan with a comparative table of species from Japanese and adjacent waters

Two new species of Spiochaetopterus (Chaetopteridae: Polychaeta), S. sanbanzensis, from Sanbanze, off Ichikawa and Funabashi Cities, Tokyo Bay, and S. izuensis from the shallow waters of Sagami Bay, were described. The most obvious difference is the number of segments in region A: 9 in S. sanbanzensis and 10 in S. izuensis. In addition, Spiochaetopterus sanbanzensis has elliptical light brown or blackish eye-spots, asymmetrical cordate specialized A4 chaetae, and a color pattern consisting of many dispersed brown spots on both ventral and dorsal faces of region A. In Spiochaetopterus izuensis, a brown band extends from each eye-spot to the level of the A1 chaetae; the convex ventral edge of the head of the specialized A4 chaeta has an oblique section and the color pattern of the body is absent. A comparison is established between these two new species and other known species from Indo-Pacific Ocean.     

Three new species of Potamothrix (Oligochaeta, Naididae, Tubificinae) from Fuxian Lake, the deepest lake of Yunnan Province, Southwest China

Three new species of Potamothrix Vejdovsky & Mrázek, 1902 (Oligochaeta: Tubificinae), Potamothrix praeprostatussp. n., Potamothrix paramoldaviensissp. n. and Potamothrix parabedotisp. n., are reported from Fuxian Lake of Yunnan Province, Southwest China. Potamothrix praeprostatusdiffers from its allies by its prostate glands joining atria in its proximal to middle portion, and spermathecal chaetae. Potamothrix paramoldaviensis is distinguishable from its allies by having penial chaeta but no penes, and differs from Potamothrix moldaviensisby its homogenous atrium. Potamothrix parabedoti is distinctive in the position of its reproductive organs, and differs from Potamothrix bedoti by its homogenous atrium. Hitherto, 34 freshwater oligochaete species have been recorded in Yunnan Province, including nine endemic species from the plateau lakes.     

Developmental stability in hybrids between the sibling species pair,Drosophila melanogaster and Drosophila simulans

Drosophila melanogaster and its sibling species D. simulans were hybridized in the laboratory to test the hypothesis that developmental homeostasis in hybrids between two species having no prior gene flow would be significantly reduced. Developmental stability was assessed by measuring fluctuating asymmetry for three bilateral traits: sternopleural chaetae, wing length, and fronto-orbital plus frontal chaetae. Male F₁ hybrids showed no decrease in developmental stability compared to males of parental species. Female hybrids showed significant fluctuating asymmetry compared to other flies. The results are discussed with respect to ideas about coadaptation and gene flow based upon previous studies of hybrid developmental stability.     

Association of allozyme heterozygosity and sternopleural chaetae number inDrosophila melanogaster

Variation at two polymorphic allozyme loci (Adh and Gpdh) has been assayed in relation to sternopleural chaetae number in flies derived from a cage population ofD. melanogaster known to be under stabilising selection with respect to chaetae number. Central phenotypes were significantly more heterozygous at these loci than more extreme phenotypes. Analysis of the base popylation and of divergent directional selection lines derived from it suggested that alleles atAdh and Gpdh have a pleiotropic effect on chaetae number or are in strong linkage disequilibrium with chaetae factors. In addition, homozygotes atAdh and Gpdh were observed to have a higher variance in chaetae number than heterozygotes. It is concluded that additive and homeostatic models of gene action might both partly explain the higher heterozygosity of central phenotypes.     

Inductive mechanisms in the process of wing vein formation inDrosophila

Summary The mechanisms underlying the formation of veins in the wing ofDrosophila have been studied by means of clonal analysis. To this end the phenotype of vein-suppression mutants (crossveinless, veinlet andradius incompletus), the vein-addition mutantplexus and a mutant causing the appearance of chaetae on the wing veins (hairy) were analysed both singly and in double mutant combinations. Subsequently various combinations of these mutants were studied in genetic mosaics. The results indicate that the cells of the dorsal surface of the wing express their genetic constitution autonomously with respect to these mutations. The ventral surface, however, has a non-autonomous behaviour with respect to vein formation but an autonomous one with respect to chaeta differentiation. Since chaeta differentiation is determined before puparium formation, i.e. before both wing surfaces some into contact, it is suggested that vein determination results from a genetic decision occurring autonomously and independently in the cells of both wing surfaces, also prior to pupariation. The final extension and course of the wing veins are modulated by cells of the dorsal surface inducing the genetically competent cells of the ventral surface.     

Egg lengths inDrosophila melanogaster and correlated responses to selection

Egg lengths were studied inDrosophila using three lines, namely a high line, where selection for increased egg length was carried out, a low line where selection for decreased egg length was carried out, and a control line. Selection was more effective in the low than the high line. Adult fly weight assayed in the 18th and 19th generations of selection responded similarly to egg length. Sternopleural chaetae number, however, responded more in the high than in the low line. In the high line the response occurred earlier than for egg length. This rather surprising result can probably be explained by assuming that the genetic architectures controlling chaetae number and egg length differ.     

Genetic heterogeneity among the founders of laboratory populations ofDrosophila. I. Scutlllar chaetae

The incidence of flies with mor than four scutellar chaetae (additional chaetae) has been followed for 45 generations in three strains set up from single inseminated females ofDrosophila melanogaster collected in the wild from the same locality at the same time. Each strain differed in the incidence of additional chaetae over this time, and the differences between strains were found to be controlled largely by additive genes.Fifteen further such strains were followed for 9 generations and each maintained consistent incidences of additional chaetae.Thus there may be genetic differences between populations derived from single inseminated females present in a given wild population, so lending support to a role for genetic drift (as a founder effect) in these situations. These differences may be maintained for many generations.Some possibilities for obtaining rapid responses in directional selection experiments based on the initial selection of favourable strains derived from single individuals are discussed briefly.