Gall-induction in insects: evolutionary dead-end or speciation driver?

Background

The tree of life is significantly asymmetrical - a result of differential speciation and extinction - but general causes of such asymmetry are unclear. Differences in niche partitioning are thought to be one possible general explanation. Ecological specialization might lead to increases in diversification rate or, alternatively, specialization might limit the evolutionary potential of specialist lineages and increase their extinction risk. Here we compare the diversification rates of gall-inducing and non-galling insect lineages. Compared with other insect herbivores feeding on the same host plant, gall-inducing insects feed on plant tissue that is more nutritious and less defended, and they do so in a favorable microhabitat that may also provide some protection from natural enemies. We use sister-taxon comparisons to test whether gall-inducing lineages are more host-specific than non-galling lineages, and more or less diverse than non-gallers. We evaluate the significance of diversity bipartitions under Equal Rates Markov models, and use maximum likelihood model-fitting to test for shifts in diversification rates.

Results

We find that, although gall-inducing insect groups are more host-specific than their non-galling relatives, there is no general significant increase in diversification rate in gallers. However, gallers are found at both extremes - two gall-inducing lineages are exceptionally diverse (Euurina sawflies on Salicaceae and Apiomorpha scale insects on Eucalytpus), and one gall-inducing lineage is exceptionally species-poor (Maskellia armored scales on Eucalyptus).

Conclusions

The effect of ecological specialization on diversification rates is complex in the case of gall-inducing insects, but host range may be an important factor. When a gall-inducing lineage has a host range approximate to that of its non-galling sister, the gallers are more diverse. When the non-galler clade has a much wider host range than the galler, the non-galler is also much more diverse. There are also lineage-specific effects, with gallers on the same host group exhibiting very different diversities. No single general model explains the observed pattern.

Tree thinking for all biology: the problem with reading phylogenies as ladders of progress

Phylogenies are increasingly prominent across all of biology, especially as DNA sequencing makes more and more trees available. However, their utility is compromised by widespread misconceptions about what phylogenies can tell us, and improved "tree thinking" is crucial. The most-serious problem comes from reading trees as ladders from "left to right"--many biologists assume that species-poor lineages that appear "early branching" or "basal" are ancestral--we call this the "primitive lineage fallacy". This mistake causes misleading inferences about changes in individual characteristics and leads to misrepresentation of the evolutionary process. The problem can be rectified by considering that modern phylogenies of present-day species and genes show relationships among evolutionary cousins. Emphasizing that these are extant entities in the 21(st) century will help correct inferences about ancestral characteristics, and will enable us to leave behind 19(th) century notions about the ladder of progress driving evolution.

Relationships among felt scale insects (Hemiptera:Coccoidea:Eriococcidae) of southern beech, Nothofagus (Nothofagaceae), with the first descriptions of Australian species of the Nothofagus-feeding genus Madarococcus Hoy

Species of southern beech (Nothofagus) have been studied extensively because of their importance in understanding southern hemisphere biogeography. Nothofagus species support a diverse assemblage of insect herbivores, including more than 30 described species of felt scales (Eriococcidae). We reconstructed the phylogeny of the Nothofagus-feeding felt scales with nucleotide sequence data and morphology. All but one of the exclusively Nothofagus-feeding species included in the analyses were recovered as a monophyletic group. This clade comprised the genera Chilechiton Hodgson & Miller, Chilecoccus Miller & González, Intecticoccus Kondo, Madarococcus Hoy (except for M. totorae Hoy), Sisyrococcus Hoy and several species of the genus Eriococcus Targioni Tozzetti. The genera Eriococcus and Madarococcus were not recovered as monophyletic. Here we revise Madarococcus. We expand the concept of the genus, provide a key to the adult females of the 31 species of Madarococcus and, for each named species, provide revised synonymies and any new collection or taxonomic information. We recognise the genus from Australia for the first time and describe the adult females of six new Australian species: Madarococcus cunninghamii Hardy & Gullan, sp. nov.; M. meander Hardy & Gullan, sp. nov.; M. megaventris Hardy & Gullan, sp. nov.; M. moorei Hardy & Gullan, sp. nov.; M. occultus Hardy & Gullan, sp. nov., and M. osculus Hardy & Gullan, sp. nov. We also describe the first-instar nymphs of M. cunninghamii, sp. nov., M. meander, sp. nov. and M. moorei, sp. nov. We transfer 17 species into Madarococcus from Eriococcus: M. argentifagi (Hoy), comb. nov.; M. cavellii (Maskell), comb. nov.; M. chilensis (Miller & González), comb. nov.; M. detectus (Hoy), comb. nov.; M. eurythrix (Miller & González), comb. nov.; M. fagicorticis (Maskell), comb. nov.; M. hispidus (Hoy), comb. nov.; M. latilobatus (Hoy), comb. nov.; M. maskelli, (Hoy), comb. nov.; M. montifagi (Hoy), comb. nov.; M. navarinoensis (Miller & González), comb. nov.; M. nelsonensis (Hoy), comb. nov.; M. nothofagi (Hoy), comb. nov.; M. podocarpi (Hoy), comb. nov.; M. raithbyi (Maskell), comb. nov.; M. rotundus (Hoy), comb. nov. and M. rubrifagi (Hoy), comb. nov. We transfer two species from Sisyrococcus into Madarococcus: M. intermedius (Maskell), comb. nov. and M. papillosus (Hoy), comb. nov. One species, M. totarae (Maskell), is excluded from Madarococcus, but cannot at present be placed in another genus and is listed as ‘M.’ totarae incertae sedis. We report the first collection of an eriococcid, M. osculus, sp. nov., on the deciduous beech, Nothofagus gunnii. With respect to biogeography, the results of our phylogenetic analysis are congruent with those obtained from recent analysis of Nothofagus; Australian and New Zealand species of Madarococcus form a monophyletic group to the exclusion of the South American species, suggesting that long-distance dispersal has played an important role in shaping the distributions of both the Nothofagus-feeding felt scales and their hosts.

A congruent molecular signature of vicariance across multiple plant lineages

Explaining disjunct distributions, or why closely related organisms are often separated by apparently severe barriers such as oceans or deserts, is a great challenge for historical biogeography. Competing explanations are long-distance dispersal across a barrier, and vicariance, in which disjunct taxa are descended from an ancestral population that was split by formation of the barrier. Vicariance explanations are testable by their prediction that near-simultaneous speciation should have occurred across multiple lineages of organisms between the disjunct areas because the origin of a barrier would potentially disrupt gene flow within multiple species. To date, there have been few studies providing evidence for multiple synchronous ancient divergences across a barrier whose origin coincides with the timing of the speciation events. Here, we use relaxed molecular-clock dating to investigate the timing of south-western (SW) versus south-eastern (SE) divergences in 23 pairs of plant lineages in southern Australia. Sixteen of the divergences correlate with the origin, 13-14 million years (Myr) ago, of the arid treeless Nullarbor Plain. The Nullarbor Plain currently forms a substantial barrier to SW-SE migration but during the last 45Myr this region has experienced multiple episodes of marine inundation and subaerial exposure. Thus, there have been multiple events that could have caused either isolation and speciation, or secondary contact, among the taxa of southern Australia. The strong molecular signal of coincident speciation in many diverse lineages during a short period provides the best evidence to date linking synchronous speciation to an ancient vicariance event.

Not so ancient: the extant crown group of Nothofagus represents a post-Gondwanan radiation

This study uses a molecular-dating approach to test hypotheses about the biogeography of Nothofagus. The molecular modelling suggests that the present-day subgenera and species date from a radiation that most likely commenced between 55 and 40 Myr ago. This rules out the possibility of a reconciled all-vicariance hypothesis for the biogeography of extant Nothofagus. However, the molecular dates for divergences between Australasian and South American taxa are consistent with the rifting of Australia and South America from Antarctica. The molecular dates further suggest a dispersal of subgenera Lophozonia and Fuscospora between Australia and New Zealand after the onset of the Antarctic Circumpolar Current and west wind drift. It appears likely that the New Caledonian lineage of subgenus Brassospora diverged from the New Guinean lineage elsewhere, prior to colonizing New Caledonia. The molecular approach strongly supports fossil-based estimates that Nothofagus diverged from the rest of Fagales more than 84 Myr ago. However, the mid-Cenozoic estimate for the diversification of the four extant subgenera conflicts with the palynological interpretation because pollen fossils, attributed to all four extant subgenera, were widespread across the Weddellian province of Gondwana about 71 Myr ago. The discrepancy between the pollen and molecular dates exists even when confidence intervals from several sources of error are taken into account. In contrast, the molecular age estimates are consistent with macrofossil dates. The incongruence between pollen fossils and molecular dates could be resolved if the early pollen types represent extinct lineages, with similar types later evolving independently in the extant lineages.

Do early branching lineages signify ancestral traits?

A reverence for ancestors that has pre-occupied humans since time immemorial persists to the present. Reconstructing ancestry is the focus of many biological studies but failure to distinguish between present-day descendants and long-dead ancestors has led to incorrect interpretation of phylogenetic trees. This has resulted in erroneous reconstruction of traits such as morphology and ancestral areas. Misinterpretation becomes evident when authors use the terms 'basal' or 'early diverging' to refer to extant taxa. Here, we discuss the correct interpretation of trees and methods for reconstructing the ancestral features of organisms using recently developed statistical models. These models can be inaccurate unless they use information that is independent of phylogenies, such as genetics, molecular and developmental biology, functional morphology, geological and climatic processes, and the fossil record.

A preliminary phylogeny of the scale insects (Hemiptera: Sternorrhyncha: Coccoidea) based on nuclear small-subunit ribosomal DNA

Scale insects (Hemiptera: Sternorrhyncha: Coccoidea) are a speciose and morphologically specialized group of plant-feeding bugs in which evolutionary relationships and thus higher classification are controversial. Sequences derived from nuclear small-subunit ribosomal DNA were used to generate a preliminary molecular phylogeny for the Coccoidea based on 39 species representing 14 putative families. Monophyly of the archaeococcoids (comprising Ortheziidae, Margarodidae sensu lato, and Phenacoleachia) was equivocal, whereas monophyly of the neococcoids was supported. Putoidae, represented by Puto yuccae, was found to be outside the remainder of the neococcoid clade. These data are consistent with a single origin (in the ancestor of the neococcoid clade) of a chromosome system involving paternal genome elimination in males. Pseudococcidae (mealybugs) appear to be sister to the rest of the neococcoids and there are indications that Coccidae (soft scales) and Kerriidae (lac scales) are sister taxa. The Eriococcidae (felt scales) was not recovered as a monophyletic group and the eriococcid genus Eriococcus sensu lato was polyphyletic.

Extraordinary and extensive karyotypic variation: a 48-fold range in chromosome number in the gall-inducing scale insect Apiomorpha (Hemiptera: Coccoidea: Eriococcidae)

Chromosome number reflects strong constraints on karyotype evolution, unescaped by the majority of animal taxa. Although there is commonly chromosomal polymorphism among closely related taxa, very large differences in chromosome number are rare. This study reports one of the most extensive chromosomal ranges yet reported for an animal genus. Apiomorpha Rübsaamen (Hemiptera: Coccoidea: Eriococcidae), an endemic Australian gall-inducing scale insect genus, exhibits an extraordinary 48-fold variation in chromosome number with diploid numbers ranging from 4 to about 192. Diploid complements of all other eriococcids examined to date range only from 6 to 28. Closely related species of Apiomorpha usually have very different karyotypes, to the extent that the variation within some species-groups is as great as that across the entire genus. There is extensive chromosomal variation among populations within 17 of the morphologically defined species of Apiomorpha indicating the existence of cryptic species-complexes. The extent and pattern of karyotypic variation suggests rapid chromosomal evolution via fissions and (or) fusions. It is hypothesized that chromosomal rearrangements in Apiomorpha species may be associated with these insects' tracking the radiation of their speciose host genus, Eucalyptus.

Extraordinary and extensive karyotypic variation: A 48-fold range in chromosome number in the gall-inducing scale insect Apiomorpha (Hemiptera: Coccoidea: Eriococcidae)

Chromosome number reflects strong constraints on karyotype evolution, unescaped by the majority of animal taxa. Although there is commonly chromosomal polymorphism among closely related taxa, very large differences in chromosome number are rare. This study reports one of the most extensive chromosomal ranges yet reported for an animal genus. Apiomorpha Rübsaamen (Hemiptera: Coccoidea: Eriococcidae), an endemic Australian gall-inducing scale insect genus, exhibits an extraordinary 48-fold variation in chromosome number with diploid numbers ranging from 4 to about 192. Diploid complements of all other eriococcids examined to date range only from 6 to 28. Closely related species of Apiomorpha usually have very different karyotypes, to the extent that the variation within some species- groups is as great as that across the entire genus. There is extensive chromosomal variation among populations within 17 of the morphologically defined species of Apiomorpha indicating the existence of cryptic species-complexes. The extent and pattern of karyotypic variation suggests rapid chromosomal evolution via fissions and (or) fusions. It is hypothesized that chromosomal rearrangements in Apiomorpha species may be associated with these insects' tracking the radiation of their speciose host genus, Eucalyptus.

Key words: Apiomorpha, cytogenetics, chromosomal evolution, holocentric.

The impact of college prematriculation immunization requirements on risk for measles outbreaks

OBJECTIVE

To assess whether prematriculation immunization requirements (PIRs) affect the number of measles cases on college campuses.

DESIGN

We surveyed a stratified random sample of 880 colleges and universities to determine their immunization policies and practices and occurrence of measles outbreaks from 1988 through 1991. We merged national measles surveillance data with survey data by county to determine the risk for measles introduction on college campuses. We used logistic regression methods to estimate the effect of PIRs and assess risk factors for college measles outbreaks.

SETTING

A total of 3205 US colleges and universities listed in standard guides.

RESULTS

Of selected schools, 91 (11%) of the 796 responding schools reported one or more measles cases occurring from 1988 through 1991. Schools with a state-mandated PIR were significantly less likely to report measles outbreaks of two or more cases than other institutions (adjusted relative risk [RR] = 0.30; 95% confidence interval [Cl], 0.11 to 0.84). None of the 14 schools that reported outbreaks of 10 or more cases was subject to state regulation or had a PIR specifying two doses of measles vaccine in place. Of schools with introduction of measles, residential colleges were more likely to report extensive spread of measles (five or more cases) than nonresidential colleges (RR = 35.8; 95% Cl, 2.08 to 617.0). Of public schools, 4-year programs had a higher risk of a large outbreak (five or more cases) than 2-year programs.

CONCLUSIONS

These results strongly support current recommendations for requiring proof of vaccination of college students to decrease the risk for measles outbreaks on college campuses. State regulations mandating PIRs ensure the best protection against widespread measles transmission.

Prematriculation immunization requirements of American colleges and universities

The authors surveyed a stratified sample of 880 colleges and universities in the United States to assess the status and characteristics of their prematriculation immunization requirements (PIRs). On the basis of a 90% return (796 responses), they estimated that 55% of US colleges and universities had implemented a PIR at the time of the survey. Among schools with PIRs, measles vaccine was almost universally required, with 74% requiring two doses, mumps vaccine was required by 70%, and rubella vaccine by 92%. Hepatitis B vaccine was rarely required and was usually recommended only for students in health-profession programs. The strongest determinant of having a PIR was the presence of a state law or regents' policy. PIRs implemented under the aegis of a state law were, on average, less comprehensive but better enforced. Other factors associated with the implementation of a PIR included membership in the American College Health Association (ACHA), the presence of a student health clinic, and availability of record-keeping personnel.

Induced sero-conversion in heifers with a field strain of bovine pestivirus--a comparison of methods and doses

Losses from pestivirus infection in a closed herd of cattle occurred over several years. In order to prevent further losses, controlled exposure of non-pregnant heifers to pestivirus from viraemic carrier animals was undertaken. Two initial experiments were conducted using either intra-nasal EDTA blood or field contact. Subsequently, other yearling heifers were inoculated with various dilutions of serum using subcutaneous, conjunctival and intra-nasal routes. Effective doses were determined. Neither inoculation nor contact infection produced any clinical illness. The highest dilutions of serum at which sero-conversion occurred were conjunctival, undiluted; intranasal, 10(-1) and subcutaneous 10(-5). With the subcutaneous route all heifers sero-converted at 10(-3). The results for the subcutaneous inoculations showed that the 50% infectious dose for cattle was not distinguishable from that determined in cell culture. Inoculation with a field strain of pestivirus in freeze-thawed serum has effectively and safely induced sero-conversion in heifers. Inoculation of all cattle at risk is considered necessary because no secondary transmission from inoculated heifers was observed.