Benjamin B. Normark

Ancient asexual scandals.

Asexual organisms that are descended from ancient asexual lineages defy current thinking on the evolution of sexual reproduction; theoreticians have been anxious to explain away their existence. However, a number of groups of organisms, from ferns to rotifers, have been suggested to be anciently asexual, and favourable evidence is being accumulated. Furthermore, new techniques for assessing claims of ancient asexuality have been proposed. Although ancient asexuals challenge current theories of sex, understanding how they manage to persist will help to explain why most organisms are sexual.

THE EVOLUTION OF AGRICULTURE IN BEETLES (CURCULIONIDAE: SCOLYTINAE AND PLATYPODINAE)

Abstract Beetles in the weevil subfamilies Scolytinae and Platypodinae are unusual in that they burrow as adults inside trees for feeding and oviposition. Some of these beetles are known as ambrosia beetles for their obligate mutualisms with asexual fungi—known as ambrosia fungi—that are derived from plant pathogens in the ascomycete group known as the ophiostomatoid fungi. Other beetles in these subfamilies are known as bark beetles and are associated with free‐living, pathogenic ophiostomatoid fungi that facilitate beetle attack of phloem of trees with resin defenses. Using DNA sequences from six genes, including both copies of the nuclear gene encoding enolase, we performed a molecular phylogenetic study of bark and ambrosia beetles across these two subfamilies to establish the rate and direction of changes in life histories and their consequences for diversification. The ambrosia beetle habits have evolved repeatedly and are unreversed. The subfamily Platypodinae is derived from within the Scolytinae, near the tribe Scolytini. Comparison of the molecular branch lengths of ambrosia beetles and ambrosia fungi reveals a strong correlation, which a fungal molecular clock suggests spans 60 to 21 million years. Bark beetles have shifted from ancestral association with conifers to angiosperms and back again several times. Each shift to angiosperms is associated with elevated diversity, whereas the reverse shifts to conifers are associated with lowered diversity. The unusual habit of adult burrowing likely facilitated the diversification of these beetle‐fungus associations, enabling them to use the biomass‐rich resource that trees represent and set the stage for at least one origin of eusociality.

Origin of a haplodiploid beetle lineage

The beetle family Scolytidae includes several groups having regular sib–mating and extremely female–biased sex ratios. Two such groups are known to include haplodiploid species: (i) the tribe Xyleborini and (ii) Coccotrypes and related genera within the tribe Dryocoetini. Relationships of these groups have been controversial. We analysed elongation factor 1–α (852 bp) and cytochrome oxidase 1 (1179 bp) sequences for 40 species. The most–parsimonious trees imply a single origin of haplodiploidy uniting Xyleborini (approximately 1200 species) and sib–mating Dryocoetini (approximately 160 species). The sister–group of the haplodiploid clade is the outcrossing genus Dryocoetes. The controversial genus Premnobius is outside the haplodiploid clade. Most haplodiploid scolytids exploit novel resources, ambrosia fungi or seeds, but a few have the ancestral habit of feeding on phloem. Thus, scolytids provide the clearest example of W. D. Hamiltons scenario for the evolution of haplodiploidy (life under bark leading to inbreeding and hence to female–biased sex ratios through haplodiploidy) and now constitute a unique opportunity to study diplodiploid and haplodiploid sister–lineages in a shared ancestral habitat. There is some evidence of sex determination by maternally inherited endosymbiotic bacteria, which may explain the consistency with which female–biased sex ratios and close inbreeding have been maintained.

Evolutionary assembly of the conifer fauna: distinguishing ancient from recent associations in bark beetles.

Several shifts from ancestral conifer feeding to angiosperm feeding have been implicated in the unparalleled diversification of beetle species. The single largest angiosperm–feeding beetle clade occurs in the weevils, and comprises the family Curculionidae and relatives. Most authorities confidently place the bark beetles (Scolytidae) within this radiation of angiosperm feeders. However, some clues indicate that the association between conifers and some scolytids, particularly in the tribe Tomicini, is a very ancient one. For instance, several fragments of Gondwanaland (South America, New Caledonia, Australia and New Guinea) harbour endemic Tomicini specialized on members of the formerly widespread and abundant conifer family Araucariaceae. As a first step towards resolving this seeming paradox, we present a phylogenetic analysis of the beetle family Scolytidae with particularly intensive sampling of conifer–feeding Tomicini and allies. We sequenced and analysed elongation factor 1α and nuclear rDNAs 18S and 28S for 45 taxa, using members of the weevil family Cossoninae as an out–group. Our results indicate that conifer feeding is the ancestral host association of scolytids, and that the most basal lineages of scolytids feed on Araucaria. If scolytids are indeed nested within a great angiosperm–feeding clade, as many authorities have held, then a reversion to conifer feeding in ancestral scolytids appears to have occurred in the Mesozoic, when Araucaria still formed a major component of the woody flora.

High diversity of structurally heterozygous karyotypes and rDNA arrays in parthenogenetic aphids of the genus Trama (Aphididae: Lachninae)

Karyotypes of permanently parthenogenetic aphids of three species of the genus Trama show great diversity, particularly in the number and distribution of chromosomal elements containing highly repetitive sequences. Sampling at only a few sites in southern England, chromosome number varied from 14 to 23 in T. troglodytes, 9–12 in T. caudata and 10–14 in T. maritima, with some colonies having individuals of more than one karyotype. This variation was paralleled by differences in the number and distribution of rDNA arrays revealed by in situ hybridization. This high intraspecific karyotype diversity contrasts with very low genetic diversity in the same populations, suggesting rapid karyotype evolution. Although T. troglodytes feeds on many species of composite plants there was no evidence of any karyotype-associated host race formation.

Incongruence Between Morphological and Mitochondrial-DNA Characters Suggests Hybrid Origins of Parthenogenetic Weevil Lineages (Genus Aramigus)

An expanded matrix of morphological characters for the genus Aramigus (Coleoptera: Curculionidae), which includes numerous polyploid parthenogenetic lineages, was compared and combined with a published matrix of mitochondrial DNA (mtDNA) characters. The matrix of morphological characters provides little resolution of the A. tessellatus and A. uruguayensis species complexes but does resolve previously unresolved relationships among other morphologically defined species (A. globoculus + A. intermedius, A. curtulus + A. planioculus). The morphological and mtDNA characters are significantly incongruent (0.435 < or = IM < or = 0.463; IMF = 0.0735), according to the tests of Farris et al. (P = 0.010) and Templeton (P < 0.005), probably because of hybrid origins of polyploid parthenogenetic lineages. For the few sexual lineages included in both matrices, morphology and mtDNA provide congruent estimates of phylogeny. In spite of recent injunctions against combining data sets that are incongruent because of differing histories, the results of the combined analyses were used to select one of the most-parsimonious mtDNA trees as the best estimate of maternal-lineage genealogy and to reconstruct the evolution of parthenogenesis under the assumption that transitions from sexuality to parthenogenesis are irreversible. Where cytogenetically justified, as in weevils, the irreversibility assumption is useful for producing conservative estimates of the age of parthenogenetic lineages in spite of potential sampling bias against sexuals.

The Strange Case of the Armored Scale Insect and Its Bacteriome

Armored scale insects are unusual in that a part of their bodies is genetically distinct from the rest. This extraordinary phenomenon challenges the notion of identity

Parthenogenesis in Insects and Mites

Publisher Summary This chapter describes parthenogenesis, a phenomenon found in some insects by which offspring can develop from an egg alone without sperm. Parthenogenetic reproduction requires a mechanism to circumvent the normal halving of ploidy that results from gametogenesis. In insects, many mechanisms for the preservation or restoration of diploidy have evolved. Either meiosis is eliminated (apomixis) or diploidy is restored (automixis) during or after meiosis. Apomixis and some forms of automixis result in maintenance of heterozygosity, whereas other forms of automixis result in instant homozygosity. Far from being a reproductive curiosity, parthenogenesis has arisen in most insect groups, and many parthenogenetic species are both abundant and widespread. Many economically important pests of agriculture and horticulture are parthenogenetic. Thelytokous and pseudogamous taxa are so ecologically successful that one cannot simply view them as reproductive aberrations. The success of parthenogenetic lineages poses something of a paradox. Either genetic variation in nature is less important than we sometimes assume, or parthenogenetic lineages are more genetically diverse than one suppose. The study of parthenogenesis can illuminate one of the central problems in biology, that of explaining the ubiquity of sex and recombination, and the adaptive significance of the laws of genetics, by revealing when and where in nature the laws of genetics are suspended or overthrown. Insects and mites, because of their short life cycles and often large population numbers, are ideal organisms for studying the evolutionary and ecological consequences of parthenogenesis.

Manuscript Variants and Old English Word Forms

Much of the Old English scribal record is redundant-important texts are often represented by several manuscripts, each slightly different from the others.1 Modern editors boil each text down to a single version, and it is this much sparser modem record of published editions on which much of the linguistic literature, including the standard grammars, is based. Comparing corresponding word forms in two manuscripts of ~Elfric’s De Temporibus Anni, I attempted to account for all the discrepancies between them using primarily Campbell’s (1959) and Brunner’s (1965) grammars. In so doing, I uncovered