Transcriptome analyses of leaf architecture in Sansevieria support a common genetic toolkit in the parallel evolution of unifacial leaves in monocots

Planar structures dramatically increase the surface-area-to-volume ratio, which is critically important for multicellular organisms. In this study, we utilize naturally occurring phenotypic variation among three Sansivieria species (Asperagaceae) to investigate leaf margin expression patterns that are associated with mediolateral and adaxial/abaxial development. We identified differentially expressed genes (DEGs) between center and margin leaf tissues in two planar-leaf species Sansevieria subspicata and Sansevieria trifasciata and compared these with expression patterns within the cylindrically leaved Sansevieria cylindrica. Two YABBY family genes, homologs of FILAMENTOUS FLOWER and DROOPING LEAF, are overexpressed in the center leaf tissue in the planar-leaf species and in the tissue of the cylindrical leaves. As mesophyll structure does not indicate adaxial versus abaxial differentiation, increased leaf thickness results in more water-storage tissue and enhances resistance to aridity. This suggests that the cylindrical-leaf in S. cylindrica is analogous to the central leaf tissue in the planar-leaf species. Furthermore, the congruence of the expression patterns of these YABBY genes in Sansevieria with expression patterns found in other unifacial monocot species suggests that patterns of parallel evolution may be the result of similar solutions derived from a limited developmental toolbox.

Regional patterning and regulation of melanin pigmentation in insects

Insects display an immense diversity in melanin pigmentation, which is generated by the interplay between the regulatory genes (that provide general patterning information) and effector genes (that provide coloration of the pattern). However, recent studies encompassing several different orders (Hemiptera, Blattodea, Coleoptera, and Lepidoptera) have shown that knockdowns of melanin producing genes alone can generate distinct region-specific patterns. This review surveys the most recent studies to further document the regional patterning of effector genes, and highlights the new advances and their implications for future research.

Splay-density coupling in semiflexible main-chain nematic polymers with hairpins

A main-chain nematic polymer melt/solution exhibits macroscopic orientational order of main polymer chains, i.e., a preferred (nematic) direction. It has long been known that in such polymeric liquid crystals spatial density/concentration variations and distortions of the nematic direction are coupled, obeying a vectorial continuity constraint whose rigidity increases with chain length. Its vectorial nature precludes the application to flexible chains, where backfolds (hairpins) are present and apolar nematic symmetry is manifest, which has been its puzzling feature from the beginning. We now establish a description of the splay-density coupling in the case of arbitrary backfolding, devising a continuity constraint for the "recovered" polar order of the chain tangents and introducing hairpins as its new type of sources. Performing detailed Monte Carlo simulations of nematic monodomain melts of "soft" worm-like chains with variable length and flexibility, we show via their structure factors that the weakening of the coupling due to the backfolding can be consistently quantified on the macroscopic level.

The contribution of the melanin pathway to overall body pigmentation during ontogenesis of Periplaneta americana

The most prominent colors observed in insects are black or brown, whose production is attributed to the melanin pathway. At present, though, the contribution of this pathway to overall body pigmentation throughout ontogenesis is still lacking. To address this question we examined the roles of 2 key melanin genes (TH and DDC), in embryonic and postembryonic development of the American cockroach, Periplaneta americana. Our results show that the melanin pathway does not contribute to the light brown coloration observed in the first nymphs. However, the dark brown coloration in mid nymphs and adults is produced solely from the melanin pathway. In addition, the DDC RNAi results reveal that it is dopamine melanin, not DOPA melanin, acts as the main contributor in this process. Overall, present study provides a new insight into insect pigmentation suggesting that genetic mechanisms of coloration can change during ontogenesis. Future studies of additional basal insect lineages will be required to assess in more details the generality of this phenomenon.

Short germ insects utilize both the ancestral and derived mode of Polycomb group-mediated epigenetic silencing of Hox genes

In insect species that undergo long germ segmentation, such as Drosophila, all segments are specified simultaneously at the early blastoderm stage. As embryogenesis progresses, the expression boundaries of Hox genes are established by repression of gap genes, which is subsequently replaced by Polycomb group (PcG) silencing. At present, however, it is not known whether patterning occurs this way in a more ancestral (short germ) mode of embryogenesis, where segments are added gradually during posterior elongation. In this study, two members of the PcG family, Enhancer of zeste (E(z)) and Suppressor of zeste 12 (Su(z)12), were analyzed in the short germ cricket, Gryllus bimaculatus. Results suggest that although stepwise negative regulation by gap and PcG genes is present in anterior members of the Hox cluster, it does not account for regulation of two posterior Hox genes, abdominal-A (abd-A) and Abdominal-B (Abd-B). Instead, abd-A and Abd-B are predominantly regulated by PcG genes, which is the mode present in vertebrates. These findings suggest that an intriguing transition of the PcG-mediated silencing of Hox genes may have occurred during animal evolution. The ancestral bilaterian state may have resembled the current vertebrate mode of regulation, where PcG-mediated silencing of Hox genes occurs before their expression is initiated and is responsible for the establishment of individual expression domains. Then, during insect evolution, the repression by transcription factors may have been acquired in anterior Hox genes of short germ insects, while PcG silencing was maintained in posterior Hox genes.

The genetic control of aposematic black pigmentation in hemimetabolous insects: insights from Oncopeltus fasciatus

Variations in body pigmentation, encompassing both the range of specific colors as well as the spatial arrangement of those colors, are among the most noticeable and lineage-specific insect features. However, the genetic mechanisms responsible for generating this diversity are still limited to several model species that are primarily holometabolous insects. To address this lack of knowledge, we utilize Oncopeltus fasciatus, an aposematic hemimetabolous insect, as a new model to study insect pigmentation. First, to determine the genetic regulation of black pigment production in Oncopeltus, we perform an RNAi analysis on three core genes involved in the melanin pathway, tyrosine hydroxylase (TH), dopa decarboxylase (DDC), and laccase 2 (lac2). The black pigmentation is affected in all instances, showing that the black pigments in this species are derived from the melanin pathway. The results of the DDC RNAi are particularly informative because they reveal that it is Dopamine melanin, not DOPA melanin, which is the predominant component of black pigments in Oncopeltus. Second, we test whether pigmentation follows a two-step model where the spatial pre-mapping of enzymatic activity is followed by vein-dependent transportation of melanin substances. We confirm the existence of the first step by observing that premature wings develop black pigmentation when exposed to melanin precursors. In addition, we provide evidence for the second step by showing that wing melanin patterning is disrupted when vein transportation is halted. These findings bring novel insights from a hemimetabolous species and establish a framework for subsequent studies on the mechanisms of pigment production and patterning responsible for variations in insect coloration.

Evolution of nubbin function in hemimetabolous and holometabolous insect appendages

Insects display a whole spectrum of morphological diversity, which is especially noticeable in the organization of their appendages. A recent study in a hemipteran, Oncopeltus fasciatus (milkweed bug), showed that nubbin (nub) affects antenna morphogenesis, labial patterning, the length of the femoral segment in legs, and the formation of a limbless abdomen. To further determine the role of this gene in the evolution of insect morphology, we analyzed its functions in two additional hemimetabolous species, Acheta domesticus (house cricket) and Periplaneta americana (cockroach), and re-examined its role in Drosophila melanogaster (fruit fly). While both Acheta and Periplaneta nub-RNAi first nymphs develop crooked antennae, no visible changes are observed in the morphologies of their mouthparts and abdomen. Instead, the main effect is seen in legs. The joint between the tibia and first tarsomere (Ta-1) is lost in Acheta, which in turn, causes a fusion of these two segments and creates a chimeric nub-RNAi tibia-tarsus that retains a tibial identity in its proximal half and acquires a Ta-1 identity in its distal half. Similarly, our re-analysis of nub function in Drosophila reveals that legs lack all true joints and the fly tibia also exhibits a fused tibia and tarsus. Finally, we observe a similar phenotype in Periplaneta except that it encompasses different joints (coxa-trochanter and femur-tibia), and in this species we also show that nub expression in the legs is regulated by Notch signaling, as had previously been reported in flies and spiders. Overall, we propose that nub acts downstream of Notch on the distal part of insect leg segments to promote their development and growth, which in turn is required for joint formation. Our data represent the first functional evidence defining a role for nub in leg segmentation and highlight the varying degrees of its involvement in this process across insects.

Pogostick: a new versatile piggyBac vector for inducible gene over-expression and down-regulation in emerging model systems

Background

Non-traditional model systems need new tools that will enable them to enter the field of functional genetics. These tools should enable the exploration of gene function, via knock-downs of endogenous genes, as well as over-expression and ectopic expression of transgenes.

Methodology

We constructed a new vector called Pogostick that can be used to over-express or down-regulate genes in organisms amenable to germ line transformation by the piggyBac transposable element. Pogostick can be found at www.addgene.org, a non-profit plasmid repository. The vector currently uses the heat-shock promoter Hsp70 from Drosophila to drive transgene expression and, as such, will have immediate applicability to organisms that can correctly interpret this promotor sequence. We detail how to clone candidate genes into this vector and test its functionality in Drosophila by targeting a gene coding for the fluorescent protein DsRed. By cloning a single DsRed copy into the vector, and generating transgenic lines, we show that DsRed mRNA and protein levels are elevated following heat-shock. When cloning a second copy of DsRed in reverse orientation into a flanking site, and transforming flies constitutively expressing DsRed in the eyes, we show that endogenous mRNA and protein levels drop following heat-shock. We then test the over-expression vector, containing the complete cDNA of Ultrabithorax (Ubx) gene, in an emerging model system, Bicyclus anynana. We produce a transgenic line and show that levels of Ubx mRNA expression rise significantly following a heat-shock. Finally, we show how to obtain genomic sequence adjacent to the Pogostick insertion site and to estimate transgene copy number in genomes of transformed individuals.

Significance

This new vector will allow emerging model systems to enter the field of functional genetics with few hurdles.

Functional analysis of Scr during embryonic and post-embryonic development in the cockroach, Periplaneta americana

The cockroach, Periplaneta americana represents a basal insect lineage that undergoes the ancestral hemimetabolous mode of development. Here, we examine the embryonic and post-embryonic functions of the hox gene Scr in Periplaneta as a way of better understanding the roles of this gene in the evolution of insect body plans. During embryogenesis, Scr function is strictly limited to the head with no role in the prothorax. This indicates that the ancestral embryonic function of Scr was likely restricted to the head, and that the posterior expansion of expression in the T1 legs may have preceded any apparent gain of function during evolution. In addition, Scr plays a pivotal role in the formation of the dorsal ridge, a structure that separates the head and thorax in all insects. This is evidenced by the presence of a supernumerary segment that occurs between the labial and T1 segments of RNAiScr first nymphs and is attributed to an alteration in engrailed (en) expression. The fact that similar Scr phenotypes are observed in Tribolium but not in Drosophila or Oncopeltus reveals the presence of lineage-specific variation in the genetic architecture that controls the formation of the dorsal ridge. In direct contrast to the embryonic roles, Scr has no function in the head region during post-embryogenesis in Periplaneta, and instead, strictly acts to provide identity to the T1 segment. Furthermore, the strongest Periplaneta RNAiScr phenotypes develop ectopic wing-like tissue that originates from the posterior region of the prothoracic segment. This finding provides a novel insight into the current debate on the morphological origin of insect wings.

Diverging functions of Scr between embryonic and post-embryonic development in a hemimetabolous insect, Oncopeltus fasciatus

Hemimetabolous insects undergo an ancestral mode of development in which embryos hatch into first nymphs that resemble miniature adults. While recent studies have shown that homeotic (hox) genes establish segmental identity of first nymphs during embryogenesis, no information exists on the function of these genes during post-embryogenesis. To determine whether and to what degree hox genes influence the formation of adult morphologies, we performed a functional analysis of Sex combs reduced (Scr) during post-embryonic development in Oncopeltus fasciatus. The main effect was observed in prothorax of Scr-RNAi adults, and ranged from significant alterations in its size and shape to a near complete transformation of its posterior half toward a T2-like identity. Furthermore, while the consecutive application of Scr-RNAi at both of the final two post-embryonic stages (fourth and fifth) did result in formation of ectopic wings on T1, the individual applications at each of these stages did not. These experiments provide two new insights into evolution of wings. First, the role of Scr in wing repression appears to be conserved in both holo- and hemimetabolous insects. Second, the prolonged Scr-depletion (spanning at least two nymphal stages) is both necessary and sufficient to restart wing program. At the same time, other structures that were previously established during embryogenesis are either unaffected (T1 legs) or display only minor changes (labium) in adults. These observations reveal a temporal and spatial divergence of Scr roles during embryonic (main effect in labium) and post-embryonic (main effect in prothorax) development.

Ubx regulates differential enlargement and diversification of insect hind legs

Differential enlargement of hind (T3) legs represents one of the hallmarks of insect evolution. However, the actual mechanism(s) responsible are yet to be determined. To address this issue, we have now studied the molecular basis of T3 leg enlargement in Oncopeltus fasciatus (milkweed bug) and Acheta domesticus (house cricket). In Oncopeltus, the T3 tibia displays a moderate increase in size, whereas in Acheta, the T3 femur, tibia, and tarsus are all greatly enlarged. Here, we show that the hox gene Ultrabithorax (Ubx) is expressed in the enlarged segments of hind legs. Furthermore, we demonstrate that depletion of Ubx during embryogenesis has a primary effect in T3 legs and causes shortening of leg segments that are enlarged in a wild type. This result shows that Ubx is regulating the differential growth and enlargement of T3 legs in both Oncopeltus and Acheta. The emerging view suggests that Ubx was co-opted for a novel role in regulating leg growth and that the transcriptional modification of its expression may be a universal mechanism for the evolutionary diversification of insect hind legs.

Analysis of nubbin expression patterns in insects

Previous studies have shown that the gene nubbin (nub) exhibits large differences in expression patterns between major groups of arthropods. This led us to hypothesize that nub may have evolved roles that are unique to particular arthropod lineages. However, in insects, nub has been studied only in Drosophila. To further explore its role in insects in general, we analyzed nub expression patterns in three hemimetabolous insect groups: zygentomans (Thermobia domestica, firebrat), dyctiopterans (Periplaneta americana, cockroach), and hemipterans (Oncopeltus fasciatus, milkweed bug). We discovered three major findings. First, observed nub patterns in the ventral central nervous system ectoderm represent a synapomorphy (shared derived feature) that is not present in other arthropods. Furthermore, each of the analyzed insects exhibits a species-specific nub expression in the central nervous system. Second, recruitment of nub for a role in leg segmentation occurred early during insect evolution. Subsequently, in some insect lineages (cockroaches and flies), this original role was expanded to include joints between all the leg segments. Third, the nub expression in the head region shows a coordinated change in association with particular mouthpart morphology. This suggests that nub has also gained an important role in the morphological diversification of insect mouthparts. Overall, the obtained data reveal an extraordinary dynamic and diverse pattern of nub evolution that has not been observed previously for other developmental genes.

Differential expression patterns of the hox gene are associated with differential growth of insect hind legs

Diversification of leg appendages is one of the hallmarks of morphological evolution in insects. In particular, insect hind (T3) legs exhibit a whole spectrum of morphological diversification, ranging from uniform to extremely modified. To elucidate the developmental basis of T3 leg evolution, we have examined the expression patterns of two homeotic genes, Ultrabithorax and abdominal-A (collectively referred to as UbdA), in a broad range of species. First, our results show that UbdA expression in hemimetabolous insects is localized only in specific T3 leg segments undergoing differential growth (compared to their foreleg counterparts). In contrast, in basal hexapod and insect lineages, the absence of the UbdA signal coincides with uniform leg morphology. The same situation exists in first instar larvae of holometabolous insects, in which absence of UbdA expression in the embryonic T3 legs is associated with the lack of larval T3 leg diversification. Second, there is a clear difference in the timing of expression between species with greatly enlarged T3 leg, such as crickets and grasshoppers, and species that exhibit more moderate enlargement of hind legs, such as mantids and cockroaches. In the former, the UbdA expression starts much earlier, coinciding with the elongation of T3 limb buds. In the latter, however, the UbdA expression starts at much later stages of development, coinciding with the establishment of distinct leg segments. These results suggest that diversification of insect hind legs was influenced by changes in both the spatial and temporal regulation of the UbdA expression.

Conservation and variation in Ubx expression among chelicerates

Chelicerates are an ancient arthropod group with a distinct body plan composed of an anterior (prosoma) and a posterior portion (opisthosoma). The expression of the Hox gene Ultrabithorax (Ubx) has been examined in a single representative of the chelicerates, the spider Cupiennius salei. In spiders, Ubx expression starts in the second opisthosomal segment (O2). Because the first opisthosomal segment (O1) in spiders is greatly reduced relative to other chelicerates, we hypothesized that the observed Ubx expression pattern might be secondarily modified. Shifts in the anterior boundary of the expression of Ubx have been correlated with functional shifts in morphology within malacostracan crustaceans. Thus, the boundary of Ubx expression between chelicerates with different morphologies in their anterior opisthosoma could also be variable. To test this prediction, we examined the expression patterns of Ubx and abdominal-A (collectively referred to as UbdA) in two basal chelicerate lineages, scorpions and xiphosurans (horseshoe crabs), which exhibit variation in the morphology of their anterior opisthosoma. In the scorpion Paruroctonus mesaensis, the anterior border of early expression of UbdA is in a few cells in the medial, posterior region of the O2 segment, with a predominant expression in O3 and posterior. Expression later spreads to encompass the whole O2 segment and a ventral, posterior portion of the O1 segment. In the xiphosuran Limulus polyphemus, early expression of UbdA has an anterior boundary in the segment. Later in development, the anterior boundary moves forward one segment to the chilarial (O1) segment. Thus, the earliest expression boundary of UbdA lies within the second opisthosomal segment in all the chelicerates examined. These results suggest that rather than being derived, the spider UbdA expression in O2 likely reflects the ancestral expression boundary. Changes in the morphology of the first opisthosomal segment are either not associated with changes in UbdA expression or correlate with late developmental changes in UbdA expression.

The value of intraoperative angiography in surgery of intracranial aneurysms: a prospective study in 126 patients

The value of intraoperative digital subtraction angiography in surgery for intracranial aneurysms, the benefits and cost-effectiveness are a matter of discussion. We prospectively studied 126 patients with 144 aneurysms, most on the anterior intracranial circulation, who underwent clipping and intraoperative angiography. Follow-up was 28.4 +/- 13.1 months. We tried to work out the indications for intraoperative angiography of the anterior circulation and its cost-effectiveness. In 10.3 % of patients (9 % per aneurysm) unexpected findings were shown by intraoperative angiography: inadequately clipped aneurysms in 10 (7.9 %), a completely unclipped aneurysm in one (0.8 %) and occluded major arteries in two (1.6 %). A broad neck was a variable of statistical significance for inadequate clipping or stenosis or occlusion of an adjacent vessel. There was a strong trend for aneurysms more than 15 mm in diameter to be "risky". Their site was not a predictive factor. We believe that intraoperative angiography is indicated in surgery not only on large and giant aneurysms, but also broad-based aneurysms of the anterior cerebral circulation regardless of their size. It is cost-effective compared to postoperative angiography. The rate of stroke in our hands was 0.8%.

The embryonic expression pattern of labial, posterior homeotic complex genes and the teashirt homologue in an apterygote insect

During embryogenesis of the fruit fly, Drosophila melanogaster, the homeotic genes are required to specify proper cell fates along the anterior-posterior axis of the embryo. We cloned partial cDNAs of homologues of the Drosophila homeotic gene teashirt and five of the homeotic-complex (HOM-C) genes from the thysanuran insect, Thermobia domestica, and assayed their embryonic expression patterns. The HOM-C genes we examined were labial, Antennapedia, Ultrabithorax, abdominal-A and Abdominal-B. As the expression pattern of these HOM-C genes is largely conserved among insects and as Thermobia is a member of a phylogenetically basal order of insects, we were able to infer their ancestral expression patterns in insects. We compare the expression patterns of the Thermobia HOM-C genes with their expression in Drosophila and other insects and discuss the potential roles these genes may have played in insect evolution. Interestingly, the teashirt homologue shows greater variability between Thermobia and Drosophila than any of the HOM-C genes. In particular, teashirt is not expressed strongly in the Thermobia abdomen, unlike Drosophila teashirt. We propose that teashirt expression has expanded posteriorly in Drosophila and contributed to a homogenization of the Drosophila larval thorax and abdomen.

Malignant prolactinoma: case report and review of the literature

BACKGROUND

Malignant prolactinomas are rare events. To date, only 14 patients with metastases in- or outside the central nervous system have been reported.

CASE DESCRIPTION

We present a patient who developed a metastasis to the cauda equina, which is the first case documented with MRI. A giant prolactinoma in this 51-year-old man was partially removed by a transcranial approach. After radiotherapy and treatment with bromocriptine, the patient had a remission for 3 years. Thereafter, a sacral intraspinal tumor was diagnosed. Because of increasing prolactin levels not responding to bromocriptine and a radiologically suspected intrasellar tumor, we operated transsphenoidally first and found only fibrous tissue. We performed a sacral laminectomy and almost totally removed an intradural tumor. Histopathology and immunohistochemistry confirmed the diagnosis of a prolactinoma metastasis. The patient received radiotherapy and bromocriptine and has no evidence of recurrent tumor or metastases after a follow-up of 38 months, thus being the second reported patient with long-term remission of the disease.

DISCUSSION

We review the literature on this topic and try to establish common features of the course of this rare malignant disease and the efficacy of therapy in the cases reported hitherto.

The expression of two engrailed-related genes in an apterygote insect and a phylogenetic analysis of insect engrailed-related genes

Homologues of the Drosophila segment polarity gene engrailed have been cloned from many insect species, as well as other arthropods and non-arthropods. We have cloned partial cDNAs of two engrailed homologues, which we call engrailed-related genes, from the phylogenetically basal insect, Thermobia domestica (Order Thysanura) and possibly as many as four engrailed-related genes from the phylogenetically intermediate insect, Oncopeltus fasciatus (Order Hemiptera). Previous to our findings, only single engrailed-related homologues had been found in phylogenetically intermediate insect species (Tribolium and Schistocerca) and in the crustacean Artemia, while two engrailed-related homologues have been found in more derived orders (Hymenoptera and the engrailed and invected genes of lepidopterans and dipterans). Consequently, we performed a phylogenetic analysis of insect engrailed-related genes to determine whether insects ancestrally had one or two engrailed-related genes. We have found evidence of concerted evolution among engrailed-related paralogues, however, that masks the true phylogenetic history of these genes; the phylogeny may only be decipherable, therefore, by examining the presence or absence of engrailed-specific and invected-specific motifs, which will require cloning the full length cDNAs from more species. In addition, we examined the embryonic expression pattern of the two Thermobia engrailed-related genes; like Drosophila engrailed and invected, they are expressed in very similar patterns, but show one temporal difference in pregnathal segments that correlates with the tentative phylogenetic placement of the genes. Thermobia engrailed-related expression also confirms that the dorsal ridge is an ancient structure in insects.

Understanding the genetic basis of morphological evolution: the role of homeotic genes in the diversification of the arthropod bauplan

Due to the segmental organization of their body plans, arthropods can be considered the paradigmatic modular organisms. In the past two decades, genetic studies of the homeotic (Hox) genes in Drosophila melanogaster have provided initial insight into the molecular mechanisms that govern the establishment of segmental identity. In this review, we will address the question of the possible role of four Hox genes: labial (lab), proboscipedia (pb), Deformed (Dfd), and Sex combs reduced (Scr) in the morphological evolution of arthropods, particularly with respect to the evolution of the head and head structures in insects. Overall, these preliminary studies illustrate the role that some of the Hox genes expressed in the insect head have played in the morphological evolution of hexapods and likely arthropods in general.

The evolutionary history of the amylase multigene family in Drosophila pseudoobscura

In Drosophila pseudoobscura, the amylase (Amy) multigene family is contained within a series of inversions, or gene arrangements, on the third chromosome. The Standard (ST), Santa Cruz (SC), and Tree Line (TL) inversions are central to the phylogeny of arrangements, and have clusters of other arrangements derived from them. The gene arrangements belonging to each of these three clusters have a characteristic number of Amy genes, ranging from three in ST to two in SC to one in TL. This distribution pattern can reflect a history of either duplications or deletions, although the data available in the past did not permit a decision between these alternatives. We provide unambiguous evidence that three Amy genes were present before the divergence of the ST, SC, and TL arrangements. Thus, the current status of the Amy multigene family is the result of deletions in the TL and SC arrangements, which created three new pseudogenes: TL Amy2-psi, TL Amy3-psi, and SC Amy3-psi. Analysis of pseudogene sequences revealed that, in the SC and ST arrangements, pseudogene evolution has been retarded, most likely due to the homogenization effect of gene conversion. Finally, by determining the original copy number, we have reconstructed the evolutionary history of the Amy multigene family and linked it with the evolution of the central gene arrangements.

Interchromosomal exchange of genetic information between gene arrangements on the third chromosome of Drosophila pseudoobscura

During the last 60 years, the inversion polymorphism on the third chromosome of Drosophila pseudoobscura has become a case study of the evolution of linked blocks of genes, isolated from each other by the suppression of recombination in heterozygotes for different inversions. Due to its location within inverted regions in most gene arrangements, the amylase (Amy) gene region can be used to elucidate the molecular pattern of evolution in these inversions. We studied this region in the Tree Line phylad of gene arrangements, with regard to both restriction site polymorphisms (RSP) and nucleotide sequences. The analysis of restriction maps, encompassing 26 kb, corroborates the cytogenetic phylogeny established on the basis of inversion breakpoints. However, we found that the 2.7 kb of nucleotide sequences of the AmyI gene are identical in both Estes Park and Hidalgo arrangements, despite the fact that these inversions arose independently from Tree Line. These contrasting results suggest that a homogenizing force, most likely gene conversion, is able to bring about localized exchanges between otherwise isolated gene arrangements.

Evidence for gene conversion in the amylase multigene family of Drosophila pseudoobscura

The alpha-amylase (Amy) multigene family in Drosophila pseudoobscura is located on the third chromosome, which is polymorphic for more than 40 inverted gene arrangements. The number of copies in this family ranges from one to three, depending on the arrangement in question. A previous study of the three Amy genes from the Standard (ST) arrangement suggested either that duplicated copies (Amy2 and Amy3) are functionally constrained or that they are undergoing gene conversion with Amy1. In order to elucidate further the pattern of molecular evolution in this family, we cloned and sequenced four additional Amy genes, two from the Santa Cruz (SC) and two from the Chiricahua (CH) gene arrangement. Of the two alternatives, only the hypothesis of gene conversion is supported by the sequence analysis. The homogenization effect of gene conversion has been strongest in SC, whose copies differ by only two nucleotides, less noticeable in ST, and negligible in the CH. Furthermore, the action of gene conversion is apparently localized, occurring only in the coding region. Interestingly, these results concur with the findings of other workers for the duplicated Amy genes in the Drosophila melanogaster group. Thus, the occurrence of gene conversion in the Amy multigene family seems to be a common feature in the Drosophila species studied so far.

The history of a genetic system

Although the chromosomal polymorphism for inversions in Drosophila pseudoobscura is one of the best studied systems in population genetics, the identity of the ancestral gene arrangement has remained unresolved for more than 50 years. There are more than 40 gene arrangements, and 4 of them (Standard, Hypothetical, Santa Cruz, and Tree Line) have been considered as candidates for the ancestral type. We propose a framework of competing hypotheses to distinguish among the alternatives. Two conclusions come from contrasting each hypothesis with the results from DNA sequencing and restriction mapping. First, not only Standard but also Hypothetical can be excluded as the ancestral gene arrangement. Second, although either Tree Line or Santa Cruz could be the ancestral type, the available data provide greater support for Santa Cruz.