Parsimonious Hypothesis Research Articles

Responses of Collicular Fixation Neurons to Gaze Shift Perturbations in Head-Unrestrained Monkey Reveal Gaze Feedback Control

A prominent hypothesis in motor control is that endpoint errors are minimized because motor commands are updated in real time via internal feedback loops. We investigated in monkey whether orienting saccadic gaze shifts made in the dark with coordinated eye-head movements are controlled by feedback. We recorded from superior colliculus fixation neurons (SCFNs) that fired tonically during fixation and were silent during gaze shifts. When we briefly (<or=700 ms) interrupted gaze shifts by transiently braking head movements, SCFNs fired steadily during brake-induced gaze immobility, and their mean frequency was inversely related to the remaining distance between current gaze position and the target. After head release, a corrective gaze saccade brought gaze on the unseen goal, and SCFN firing frequency peaked. The results support gaze feedback control and show that the SC is part of a network that encodes, during orientation, the distance between eye and target, irrespective of gaze trajectory characteristics.

Early dinosaurs: A phylogenetic study

Synopsis Early dinosaur evolution has been the subject of several phylogenetic studies and the position of certain basal forms is currently debated. This is the case for the oldest known members of the group, excavated from the Late Triassic Ischigualastian beds of South America, such as Herrerasaurus, Eoraptor, Pisanosaurus, Saturnalia and Staurikosaurus. A new cladistic analysis of the early dinosaur radiation was performed to assess the relationships among the three major clades (Ornithischia, Sauropodomorpha and Theropoda) and to define the phylogenetic position of the basal members of the group. The most parsimonious hypothesis has Silesaurus opolensis as the sister taxon to a dichotomy including monophyletic Saurischia and Ornithischia. The latter includes Pisanosaurus mertii, and the former all other well‐known Triassic dinosaurs. Saurischia is composed of two major monophyletic groups: Herrerasauridae (including Herrerasaurus ischigualastensis and Staurikosaurus pricei) and Eusaurischia (including the theropod and sauropodomorph lineages), while Eoraptor lunensis appears to represent the sister taxon to Eusaurischia. Saturnalia tupiniquim is a stem‐taxon to Sauropodomorpha and Guaibasaurus candelariensis might belong to the theropod branch. Some of these hypotheses are, however, not strongly supported. Especially uncertain are the affinities of Silesaurus and Guaibasaurus. The latter can only be safely regarded as a saurischian, while the former might belong to the ornithischian lineage. The dinosaurian affinities of Eoraptor and Herrerasauridae are strongly supported. Yet, the possibility that they (especially Eoraptor) represent basal theropods, rather than basal saurischians, cannot be dismissed. In fact, basal saurischian evolution is still too poorly understood for a definitive hypothesis of relationships to be presented.

Eremotherium Laurillardi (Lund) (Xenarthra, Megatheriidae), the Panamerican giant ground sloth: Taxonomic aspects of the ontogeny of skull and dentition

Synopsis For most of the past 150 years the presence of only giant megatheriine ground sloths in the Pleistocene of Brazil has been recognised. However, it has recently been claimed that a dwarf Eremotherium species was present sympatrically with the large‐sized giant ground sloth in northeastern and southeastern Brazil during the late Pleistocene and that the concept of a single Panamerican species was improbable. Based on analysis of the ontological sequence of skull suture closure and dental development of remains from a single locality, we demonstrate that the ‘dwarf’ species is based on the remains of immature individuals, at very early ontogenetic stages, of the large‐sized species. Further, we reaffirm that the most parsimonious hypothesis is to consider all large‐sized Eremotherium remains as belonging to a single species of Panamerican distribution.

Review of fungus gnats of the genus Tarnania Tuomikoski, with a phylogeny of the Rymosia s.l. genus group (Diptera: Mycetophilidae)

AbstractThe four known species of Tarnania Tuomikoski are reviewed, viz. T. dziedzickii (Edwards, 1941), T. fenestralis (Meigen, 1818), T. nemoralis (Edwards, 1941), and T. tarnanii (Dziedzicki, 1910). Males and females are re-described, photographed and their terminalia figured. An attempt is made to homologize and name specific substructures of the male gonostylus, widely recognizable within the tribe Exechiini. A phylogenetic analysis of the Tarnania species, including representatives of the six other genera of the Rymosia s. lat. genus group, strengthens the view that Tarnania is monophyletic. Tarnania is supported by six unambiguous character changes of which three are considered to be unique synapomorphies, viz. 1) lower parts of metepisternum and metepimeron covered with campaniform sensilla; 2) hind tibia with several curved bristles posterodorsally, placed irregularly in more than one row; and 3) anterior branch of male gonostylus forming an elongated, bifurcated lobe. The most parsimonious hypothesis for relationships among the species of Tarnania is ((T. tarnanii + T. nemoralis) + (T. dziedzickii + T. fenestralis)). The practice of regarding the genera of the Rymosia s. lat. genus group as an assemblage of plesiomorphic genera is also supported, the most parsimonious hypothesis for relationships among them being (Notolopha (Allodiopsis (Myrosia (Synplasta (Tarnania + (Rymosia + Pseudorymosia)))))). Tarnania is most diverse in northwestern Europe, with all four species known from Norway, Sweden, Great Britain, Belgium, Germany, Switzerland, and France. T. dziedzickii extends its distribution southwards to include the North African and the Near East Regions. T. fenestralis is widely distributed in the western Palaearctic Region and extends its distribution eastwards to the eastern Palaearctic region. Only T. tarnanii displays a wide Holarctic distribution including most of Russia, Alaska, Canada and Greenland.

The family Alestidae (Ostariophysi, Characiformes): a phylogenetic analysis of a trans-Atlantic clade

The overall most parsimonious hypothesis of relationships based on 200 characters indicates that the Alestidae is the closest relative of Chalceus, a genus previously assigned to the Neotropical Characidae. Chalceus is shifted into the Alestidae, which becomes the only trans-Atlantic family level group within the Characiformes. Various previously proposed suprageneric assemblages within the Alestidae (e.g. Petersiini) failed to delimit monophyletic groups under the intrafamilial phylogenetic analysis. The evaluation of fossil alestids within the context of the phylogeny indicates that the ancestors of Alestes, Arnoldichthys, Brycinus, Bryconaethiops and Hydrocynus evolved prior to the early Eocene (Cuisian of Upper Ypresian), 49–54.8 million years ago, with the fossil Alestoides most closely related to Alestes. The phylogenetic information further indicates a minimum age of 90–112 million years for the Alestidae. Contrary to previous hypotheses, the fossil African Sindacharax was found to be most similar to the clade including the alestid genus Bryconaethiops rather than most closely related to the South American subfamily Serrasalminae. Evaluation of the fossil Mahengecharax carrolli fails to support its hypothesized placement as the sister group to all Recent members of the Alestidae. Two separate episodes of miniaturization and one episode of gigantism occurred within the evolution of the Alestidae. © 2005 The Linnean Society of London, Zoological Journal of the Linnean Society, 2005, 145, 1−144. No claim to original US government works.

The evolution of a Müllerian mimic in a spatially distributed community

Strong positive density-dependence should lead to a loss of diversity, but warning-colour and Müllerian mimicry systems show extraordinary levels of diversity. Here, we propose an analytical model to explore the dynamics of two forms of a Müllerian mimic in a heterogeneous environment with two alternative model species. Two connected populations of a dimorphic, chemically defended mimic are allowed to evolve and disperse. The proportions of the respective model species vary spatially. We use a nonlinear approximation of Müller's number-dependent equations to model a situation where the mortality for either form of the mimic decreases hyberbolically when its local density increases. A first non-spatial analysis confirms that the positive density-dependence makes coexistence of mimetic forms unstable in a single isolated patch, but shows that mimicry of the rarer model can be stable once established. The two-patch analysis shows that when models have different abundance in different places, local mimetic diversity in the mimic is maintained only if spatial heterogeneity is strong, or, more interestingly, if the mimic is not too strongly distasteful. Therefore, mildly toxic species can become polymorphic in a wider range of ecological settings. Spatial dynamics thus reveal a region of Müllerian polymorphism separating classical Batesian polymorphism and Müllerian monomorphism along the mimic's palatability spectrum. Such polymorphism-palatability relationship in a spatial environment provides a parsimonious hypothesis accounting for the observed Müllerian polymorphism that does not require quasi-Batesian dynamics. While the stability of coexistence depends on all factors, only the migration rate and strength of selection appear to affect the level of diversity at the polymorphic equilibrium. Local adaptation is predicted in most polymorphic cases. These results are in very good accordance with recent empirical findings on the polymorphic butterflies Heliconius numata and H. cydno.

Segregation patterns of isozyme loci and microsatellite markers show the diploidy of African yam Dioscorea rotundata (2n=40)

The cultivated yam species Dioscorea rotundata (2n = 40) has been considered by most authors as a tetraploid species with a basic chromosome number of ten. In this paper, we analysed the segregation of two isozyme loci and six microsatellite markers in the progeny of a self-fertilised monoecious plant. For the eight markers, segregation patterns could be explained by only two genetic models: diploidy or tetraploidy with two null alleles. Given the nature of studied markers, the most parsimonious hypothesis was that the parental plant was diploid. These results, data from a diversity survey and results of other authors led to the conclusion that D. rotundata is a diploid species.

Filling in the gaps: EST surveys of under‐represented chromalveolates

The “chromalveolate hypothesis” posits the monophyly of alveolates (apicomplexans, dinoflagellates and ciliates), heterokonts, haptophytes and cryptomonads. One implication of the monophyly of chromalveolates is that the plastids in this group could share a common ancestor—derived from a single endosymbiosis. Members of all chromalveolate groups, with the exception of ciliates, possess a secondary plastid originating from a rhodophytic symbiont (although the rhodophytic origin of the apicomplexan plastid remains contentious). A single origin of this chromalveolate plastid represents the most parsimonious hypothesis for plastid acquisition. Growing support for the “chromalveolate hypothesis” has relied on a relatively small number of molecular studies to date, and a more thorough examination of this alliance has been undermined by a paucity of broad molecular data from several of its members, despite complete genome sequences from apicomplexans and ciliates. To address this deficit we have undertaken EST (Expressed Sequence Tag) surveys of several taxa, representing dinoflagellates, haptophytes and cryptomonads. Using EST data we are undertaking broad molecular comparisons of these groups through molecular phylogenies, analyses of common biochemical pathways, and studies of organelle evolution. This genome‐based approach provides a more complete view of the major chromalveolate groups, and a chance to more thoroughly test the chromalveolate hypothesis.

Phosphatidylinositols as Ligands for Orphan Nuclear Receptors?

Phosphatidylinositol lipids are implicated in a broad range of signaling processes, from organization of signaling pathways to vesicle trafficking and control of the actin cytoskeleton. Now, structural studies by Krylova et al . strongly suggest that these lipids may also serve as activating ligands for a class of orphan (so-called because no regulatory ligand was known) nuclear receptors. The authors focused on members of the nuclear receptor 5A subfamily and obtained crystal structures of family members, including mouse mSF-1 and the human proteins hSF-1 and hLRH-1. The proteins showed protein folds characteristic of the ligand-binding domains of other nuclear receptors that are regulated by known ligands. Electron density in the ligand-binding pocket revealed that the bacterially expressed proteins contained lipids. The authors therefore tested binding to lipids from eukaryotic cells and detected preferential binding to phosphatidylinositol 3,5-bisphosphate [PI(3,5)P 2 ] and phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P 3 ]. Although biological regulation by such lipids remains to be fully explored, mutant proteins designed to disrupt lipid binding had decreased transcriptional activity in a luciferase reporter assay in human HepG2 cells. The rodent receptor appears to have lost ligand binding, and the authors' evolutionary analysis indicated that the most parsimonious hypothesis is that ancestral proteins did bind lipid ligands and that this property was later lost in the rodent protein. The authors note that an ancestral orphan receptor may not have gained ligand binding over the course of evolution, as previously proposed, but rather there may have been repeated loss of ligand binding from a ligand-regulated ancestral protein. I. N. Krylova, E. P. Sablin, J. Moore, R. X. Xu, G. M. Waitt, J. A. MacKay, D. Juzumiene, J. M. Bynum, K. Madauss, V. Montana, L. Lebedeva, M. Suzawa, J. D. Williams, S. P. Williams, R. K. Guy, J. W. Thornton, R. J. Fletterick, T. M. Willson, H. A. Ingraham, Structural analyses reveal phosphatidyl inositols as ligands for the NR5 orphan receptors SF-1 and LRH-1. Cell 120 , 343-355 (2005). [PubMed]

When and where did troidine butterflies (Lepidoptera : Papilionidae) evolve? Phylogenetic and biogeographic evidence suggests an origin in remnant Gondwana in the Late Cretaceous

The age, geographic origin and time of major radiation of the butterflies (Hesperioidea + Papilionoidea + Hedyloidea) are largely unknown. The general modern view is that butterflies arose during the Late Jurassic/Cretaceous in the southern hemisphere (southern Pangea/Gondwana before continental breakup), but this is not universally accepted, and is a best guess based largely on circumstantial evidence. The extreme paucity of fossils and lack of modern, higher-level phylogenies of extant monophyletic groups have been major impediments towards determining reliable estimates of either their age or geographic origin. Here we present a phylogenetic and historical biogeographic analysis of a higher butterfly taxon, the swallowtail tribe Troidini. We analysed molecular data for three protein-encoding genes, mitochondrial ND5 and COI–COII, and nuclear EF–1α, both separately and in combination using maximum parsimony (with and without character weighting and transition/transversion weighting), maximum likelihood and Bayesian methods. Our sample included representatives of all 10 genera of Troidini and distant ingroup taxa (Baroniinae, Parnassiinae, Graphiini, Papilionini), with Pieridae as outgroup. Analysis of the combined dataset (4326 bp; 1012 parsimony informative characters) recovered the Troidini as a well supported monophyletic group and the monophyly of its two subtribes, Battina and Troidina. The most parsimonious biogeographic hypothesis suggests a southern origin of the tribe in remnant Gondwana (Madagascar–Greater India–Australia–Antarctica–South America) sometime after the rifting and final separation of Africa in the Late Cretaceous (&lt;90 Mya). Although an ancient vicariance pattern is proposed, at least four relatively recent dispersal/extinction events are needed to reconcile anomalies in distribution, most of which can be explained by geological and climatic events in South-east Asia and Australia during the late Tertiary. Application of a molecular clock based on a rate smoothing programme to estimate various divergence times based on vicariance events, revealed two peculiarities in our biogeographic vicariance model that do not strictly accord with current understanding of the temporal breakup of Gondwana: (1) the troidine fauna of Greater India did not become isolated from Gondwana (Antarctica) until the end of the Cretaceous (c. 65 Mya), well after Madagascar separated from Greater India (84 Mya); and (2) the faunas of Greater India, Australia and South America diverged simultaneously, also at the K/T boundary. A recent published estimate of the time (31 Mya) of divergence between Cressida Swainson (Australia) and Euryades Felder &amp; Felder (South America) is shown to be in error.

Evolutionary relationships between 15 Plasmodium species from New and Old World primates (including humans): a 18S rDNA cladistic analysis

We present a new phylogenetic analysis of 15 primate Plasmodium species based on 18S rDNA sequences including new sequences of Plasmodium coatneyi, P. fieldi, P. gonderi, P. hylobati and P. simium. The results are discussed in the context of the parasite host species and their geographical distribution. Contrary to other phylogenies constructed with this 18S rDNA molecule, we observed that the topology of phylogenetic trees was not affected either by the quality of the nucleotide matrices, or by the species present in the outgroup. This analysis showed the following. (1) The polyphyly of human Plasmodium is confirmed. (2) The monophyly of Plasmodium from Old World monkeys is confirmed by the new added sequences and P. gonderi, an African species, possibly could be at the root of this group. (3) The most parsimonious biogeographical hypothesis is that P. vivax originated in Asia; thus, its related species P. simium appears to be derived through a transfer from the human P. vivax to New World monkey species in South America. (4) Sampling efforts of non-human primate Plasmodium could permit improvement of the knowledge of primate Plasmodium phylogeny and also consideration of the risks of malaria emergence from monkey reservoirs.

Genetic diversity, clonality and sexuality in Toxoplasma gondii

The majority of Toxoplasma gondii strains from a variety of human and animal sources have been grouped into three highly clonal but closely related lineages. The low occurrence of nucleotide differences among the three predominant lineages and their unusual dimorphic allelic composition suggest that they have arisen from a recent common ancestry. Less than 1% of the previously studied strains contain unique genotypes and high divergence of DNA sequence, and therefore are considered ‘exotic’ or ‘atypical’ strains. The seemingly low genetic diversity in T. gondii may have been underestimated because most parasite strains in previous studies were collected from human patients and domestic animals in North America and Europe. To investigate the genetic diversity of T. gondii, we analysed parasite strains isolated from remote geographical regions by multilocus microsatellite sequencing and phylogenetic analysis. The genetic diversity indices, the molecular analysis of microsatellite genotypes and the constructed phylogram considered together suggest that the global T. gondii population is highly diversified and not characteristic of a clonal organism. The most parsimonious hypothesis is that T. gondii presents a complex population structure with a mix of clonal and sexual propagation as a function of the environmental conditions. The comparison between domestic strains data on one hand and wild strains data on the other hand is in favour of more frequent sexual recombinations in wild environment even though Toxoplasma subpopulation in human and domestic animals is largely clonal.

Phylogeny of the Lonchophyllini (Chiroptera: Phyllostomidae)

A combination of 1,140 base pairs of the mitochondrial cytochrome b gene of Platalina, Lionycteris, and several species of Lonchophylla (Chiroptera: Phyllostomidae) with 150 morphological, sex chromosome, and restriction site characters were used in an attempt to resolve relationships among the lonchophylline taxa. In addition, the monophyly of Lonchophylla was tested, particularly with respect to Platalina. The most parsimonious hypothesis of relationships using all available characters was (L. mordax ((L. chocoana (L. robusta, L. handleyi))(L. thomasi (Lionycteris, Platalina)))). Lonchophylla appears to be paraphyletic, but this arrangement is not well supported. Our analyses suggest that Platalina is not simply a large Lonchophylla, as had been suggested by previous morphological analyses. The low support values for basal relationships found in this study are probably caused by saturation in cytochrome b 3rd positions. Additionally, 2 alternative explanations are viable (if improbable): unsampled lonchophyllines are necessary to confidently resolve relationships at the base of the group, or the lack of resolution at the base of the lonchophylline phylogeny might be explained by rapid speciation following the separation from other glossophagines. Future work examining the phylogenetic relationships of lonchophylline bats should focus on describing new taxa, obtaining tissue samples from unsequenced representatives, and adding nuclear loci to this mitochondrial DNA data set.

PHYLOGENY OF THE LONCHOPHYLLINI (CHIROPTERA: PHYLLOSTOMIDAE)

A combination of 1,140 base pairs of the mitochondrial cytochrome b gene of Platalina, Lionycteris, and several species of Lonchophylla (Chiroptera: Phyllostomidae) with 150 morphological, sex chromosome, and restriction site characters were used in an attempt to resolve relationships among the lonchophylline taxa. In addition, the monophyly of Lonchophylla was tested, particularly with respect to Platalina. The most parsimonious hypothesis of relationships using all available characters was (L. mordax ((L. chocoana (L. robusta, L. handleyi))(L. thomasi (Lionycteris, Platalina)))). Lonchophylla appears to be paraphyletic, but this arrangement is not well supported. Our analyses suggest that Platalina is not simply a large Lonchophylla, as had been suggested by previous morphological analyses. The low support values for basal relationships found in this study are probably caused by saturation in cytochrome b 3rd positions. Additionally, 2 alternative explanations are viable (if improbable): unsampled lonchophyllines are necessary to confidently resolve relationships at the base of the group, or the lack of resolution at the base of the lonchophylline phylogeny might be explained by rapid speciation following the separation from other glossophagines. Future work examining the phylogenetic relationships of lonchophylline bats should focus on describing new taxa, obtaining tissue samples from unsequenced representatives, and adding nuclear loci to this mitochondrial DNA data set.

How much can cladistics tell us about early hominid relationships?

Although cladistic analysis has been used to compare hypotheses of relationships among early hominids, the outcomes of different studies have depended entirely on the assumptions made by different investigators. Problems include the close genetic relationship of early hominid taxa, small fossil sample sizes, possible correlations among characters, and a lack of understanding about the evolutionary factors affecting characters. This study investigates the interaction of some of these problems affecting early hominid phylogenetics. Monte Carlo simulations of character state evolution in closely related taxa demonstrate that the sample sizes and close genetic relationships of early hominids do not permit cladistic analyses to obtain unequivocal results. Even with unrealistically good assumptions about the evolutionary dynamics affecting characters, the probability of the most parsimonious hypothesis being true is unacceptably small. In the face of these problems, even phylogenetic statements that are supported by a strong consensus of cladistic studies may nevertheless be in error, and such errors are likely to confound the placement of new specimens and taxa. Advancement in our knowledge of hominid phylogeny can depend only on a fuller understanding of the natural history and evolutionary dynamics of traits.

History of West Mediterranean newts, Pleurodeles (Amphibia: Salamandridae), inferred from old and recent DNA sequences

MtDNA sequences (396 bp cytochrome b and 369 bp 12S rRNA) from recent material and old museum specimens indicate Pleurodeles poireti and P. waltl form independent clades with 7.76% genetic divergence. Within P. poireti, populations from Djebel Edough, NE Algeria are very distinct with 6.12% genetic divergence from the remainder and may deserve separate species status. Away from Djebel Edough, P. poireti consists of three distinct clades (coastal NW Tunisia; central N Algeria; Constantine plus inland NW Tunisia) with a maximum genetic divergence of only 1%. P. waltl contains two clades with 2.96% genetic divergence, one in SE and E Spain plus north Morocco, the other in Portugal and SW and central Spain. Pleurodeles probably invaded NW Africa from SW Europe during the Messinian Salinity Crisis, when land contact was first established at 5.6 Ma, and then interrupted at 5.3 Ma. Molecular clocks, calibrated in the assumption that the latter event separated P. waltl and P. poireti, suggest that Pleurodeles diverged from its sister taxon, Tylototriton, at about 8.6–10 Ma. Djebel Edough P. poireti separated at about 4.2 Ma, perhaps through isolation on a temporary, now ‘fossil’, island initiated by the Messinian crisis. Differentiation in remaining P. poireti may have been caused by Pleistocene climatic fluctuations, while bifurcation in P. waltl appears to have taken place in the Pliocene approximately between 3.2 and 2 Ma. This species reached Morocco very recently, perhaps as a result of human introduction. Use in Pleurodeles of the slower divergence rates estimated in some other salamandrids results in a less parsimonious historical hypothesis that does not fit known geophysical events.

Origin and spread of the 1278insTATC mutation causing Tay-Sachs disease in Ashkenazi Jews: genetic drift as a robust and parsimonious hypothesis.

The 1278insTATC is the most prevalent beta-hexosaminidase A ( HEXA) gene mutation causing Tay-Sachs disease (TSD), one of the four lysosomal storage diseases (LSDs) occurring at elevated frequencies among Ashkenazi Jews (AJs). To investigate the genetic history of this mutation in the AJ population, a conserved haplotype (D15S981:175-D15S131:240-D15S1050:284-D15S197:144-D15S188:418) was identified in 1278insTATC chromosomes from 55 unrelated AJ individuals (15 homozygotes and 40 heterozygotes for the TSD mutation), suggesting the occurrence of a common founder. When two methods were used for analysis of linkage disequilibrium (LD) between flanking polymorphic markers and the disease locus and for the study of the decay of LD over time, the estimated age of the insertion was found to be 40+/-12 generations (95% confidence interval: 30-50 generations), so that the most recent common ancestor of the mutation-bearing chromosomes would date to the 8th-9th century. This corresponds with the demographic expansion of AJs in central Europe, following the founding of the Ashkenaz settlement in the early Middle Ages. The results are consistent with the geographic distribution of the main TSD mutation, 1278insTATC being more common in central Europe, and with the coalescent times of mutations causing two other LSDs, Gaucher disease and mucolipidosis type IV. Evidence for the absence of a determinant positive selection (heterozygote advantage) over the mutation is provided by a comparison between the estimated age of 1278insTATC and the probability of the current AJ frequency of the mutant allele as a function of its age, calculated by use of a branching-process model. Therefore, the founder effect in a rapidly expanding population arising from a bottleneck provides a robust parsimonious hypothesis explaining the spread of 1278insTATC-linked TSD in AJ individuals.

Karyotype description of Pomacea patula catemacensis (Caenogastropoda, Ampullariidae), with an assessment of the taxonomic status of Pomacea patula

Mitotic chromosomes of the freshwater snail Pomacea patula catemacensis (Baker 1922) were analyzed on gill tissue of specimens from the type locality (Lake Catemaco, Mexico). The diploid number of chromosomes is 2n = 26, including nine metacentric and four submetacentric pairs; therefore, the fundamental number is FN = 52, No sex chromosomes could be identified. The same chromosome number and morphology were already reported for P. flagellata, i.e., the other species of the genus living in Mexico. The basic haploid number for family Ampullariidae was reported to be n = 14 in the literature; so, its reduction to n = 13 is probably an apomorphy of the Mexican Pomacea snails. Lanistes bolteni, from Egypt, also shows n = 13, but its karyotype is much more asymmetrical, and seems to have evolved independently from P. flagellata and P. patula catemacensis. The nominotypical subspecies, P. patula patula (Reeve 1856), is a poorly known taxon, whose original locality is unknown. A taxonomical account is presented here, and a Mexican origin postulated as the most parsimonious hypothesis.

Morphological appraisal of Collembola phylogeny with special emphasis on Poduromorpha and a test of the aquatic origin hypothesis

Phylogenetic relationships within Collembola were determined through the cladistic analysis of 131 morphological characters and 67 exemplar taxa representing the major families of the group, with special emphasis on Poduromorpha. The results show that the order Poduromorpha is monophyletic and the sister group to the remaining Collembola, with Entomobryomorpha monophyletic and the sister group to the clade Neelipleona + Symphypleona. In Entomobryomorpha, Actaletidae is the sister group of the remaining families. In Poduromorpha, Tullbergiinae is monophyletic as well as Onychiurinae and the group Tetrodontophorinae + Onychiurinae which is the sister group of the remaining Poduromorpha; Tetrodontophorinae is paraphyletic; Onychiuridae is polyphyletic; Isotogastruridae is not an intermediate between Poduromorpha and Entomobryomorpha, it is the sister group of Tullbergiinae; Odontellidae is monophyletic and the sister group to the clade Neanuridae + Brachystomellidae; in Neanuridae, Frieseinae and the group Pseudachorutinae + Morulinae + Neanurinae are monophyletic; Morulinae is the sister group of Neanurinae; Pseudachorutinae is paraphyletic; Hypogastruridae is polyphyletic;Podura aquatica(Poduridae) is not ‘primitive’, it clusters with the generaXenyllaandParaxenyllain Hypogastruridae. On the basis of these relationships and the position of the aquatic species, the most parsimonious hypothesis is a terrestrial edaphic origin for the springtails.

Time-varying, slow-phase component interaction in congenital nystagmus

We investigated the nystagmus of a 12-year-old boy with suspected X-linked congenital nystagmus (CN) and exophoria to determine the underlying mechanisms and component signals in the ‘dual-velocity’ and other slow phases of his Asymmetric (a)Periodic Alternating Nystagmus (APAN). Fast Fourier transforms (FFT) were performed on the waveforms and residual data after subtracting a sawtooth waveform whose amplitude and frequency matched those of the jerk nystagmus. The FFT analyses identified two frequency components (jerk––4 Hz and pendular––4 and 8 Hz, variable) that varied differently in intensity and frequency/phase over the time-course of the APAN. We synthesized each of the patient’s slow phases using summation of sawtooth and sinusoidal waveforms. The resulting waveforms included jerk (with different slow-phase appearances), dual jerk, and pendular. We demonstrated that the pendular nystagmus seen during the neutral phase of APAN and the appearance of either decelerating (mimicking latent nystagmus), dual-velocity, or dual-jerk slow phases can be explained and produced by the summation of linear and pendular components of variable amplitudes and frequencies/phases. Thus, one mechanism may be responsible for all the variation seen in this patient’s slow phases, rather than the less parsimonious hypothesis of a switched-tonic-imbalance mechanism that we had originally suggested to simulate the dual-velocity waveform.