Investigation of the population structure of the tick vector of Lyme disease Ixodes scapularis (Acari: Ixodidae) in Canada using mitochondrial cytochrome C oxidase subunit I gene sequences.

Genotyping of Ixodes scapularis Say (Acari: Ixodidae) ticks could enhance understanding of the occurrence and genotypes of I. scapularis-borne pathogens. We investigated the utility of mitochondrial (mt) Cytochrome C Oxidase subunit I gene (cox1) sequences as a tool for understanding the population structure of I. scapularis collected in Canada, where we also investigated the geographic occurrence of different cox1 haplotypes. Sequences obtained from 414 ticks were one of 55 unique haplotypes, most of which grouped into one of six clades. Demographic analysis suggested that cox1 sequences have haplotype and nucleotide diversity comparable to other mt genes. All haplotypes were connected in a single minimum spanning network tree. Despite low fixation index values there were significant differences in the frequency of occurrence of haplotypes of different clades among four geographic regions: 1) Alberta to western Ontario, 2) eastern Ontario, 3) Quebec, and 4) Atlantic Provinces; suggesting that cox1 sequences could reveal population structure differences between I. scapularis in geographically separated populations of northeastern and midwestern North America. Spatial clusters of ticks of the same haplotype identified in regions of southern Quebec and southern Ontario where I. scapularis is invading were consistent with population bottlenecks associated with founder events. These findings suggest that cox1 sequences are useful for the study of I. scapularis population structure, are of sufficient diversity that spatial analyses of haplotypes can be used to identify where I. scapularis is emerging in southern Canada, and may be useful for exploring differences between northeastern and midwestern populations of I. scapularis.

Identifying the last supper: utility of the DNA barcode library for bloodmeal identification in ticks.

Ticks are among the most important vectors of disease in the Northern Hemisphere, and a better understanding of their feeding behaviour and life cycle is critical to the management and control of tick-borne zoonoses. DNA-based tools for the identification of residual bloodmeals in hematophagous arthropods have proven useful in the investigation of patterns of host use in nature. Using a blind test approach, we challenged the utility of the DNA barcode library for the identification of vertebrate bloodmeals in engorged, field-collected Ixodes scapularis. Universal vertebrate primers for the COI barcode region successfully amplified DNA from the host bloodmeal and only rarely amplified tick DNA. Of the 61 field-collected ticks, conclusive genus- and species-level identification was possible for 72% of the specimens. In all but two cases, barcode-based identification of the bloodmeal was consistent with the morphological identification of the vertebrate host the ticks were collected from. Possible explanations for mismatches or ambiguities are presented. This study validates the utility of the DNA barcode library as a valuable and reliable resource for the identification of unknown bloodmeals in arthropod vectors of disease. Future directions aimed at the refinement of these techniques to gain additional information and to improve the amplification success of digested vertebrate DNA in tick bloodmeals are discussed.

An expansion of the genome size dataset for the insect order Hymenoptera, with a first test of parasitism and eusociality as possible constraints.

Although the Hymenoptera represent a remarkably diverse and socioeconomically important group that is of considerable interest in genome biology, they remain understudied in terms of genome size. This study reports new genome size estimates for 89 species of ants, bees and wasps, representing 17 families and four superfamilies. These are used in a test of the hypothesis that genome sizes are constrained by traits associated with parasitism or eusociality. Not all parasitoid wasps exhibit small genomes, though a relationship based on specific types of parasitism may still occur; by contrast, there was no convincing evidence of a constraint relating to eusociality. The data provided here can be used to guide future research aimed at understanding the evolution of large-scale genomic properties in this order.

Genome size diversity in the family Drosophilidae.

Flies in the genus Drosophila have been the dominant model organisms in genetics for over a century and, with a dozen complete sequences now available, continue as such in modern comparative genomics. Surprisingly, estimates of genome size for this genus have been relatively sparse, covering less than 2% of species. Here, best practice flow cytometric genome size estimates are reported for both male and female flies from 67 species from six genera in the family Drosophilidae, including 55 species from the genus Drosophila. Direct and phylogenetically corrected correlation analyses indicate that genome size is positively correlated with temperature-controlled duration of development in Drosophila, and there is indication that genome size may be positively related to body size and sperm length in this genus. These findings may provide some explanation for the streamlined genomes found in these insects, and complement recent work demonstrating possible selective constraints on further deletion of noncoding DNA.

Genome sizes of spiders.

Spiders represent a diverse and familiar group of animals, but to date no information has been made available regarding their genome sizes. Arachnids in general have been almost entirely overlooked, and are currently represented by a single tick in the animal genome size data set. The present study provides new genome size estimates for 115 species of spiders from 19 families (all from the infraorder Araneomorphae), thereby adding considerably to the coverage of these arthropods. No clear-cut patterns of variation were detectable even with this relatively large data set, but some interesting avenues for future research have been illuminated by this preliminary survey.

Temporal control of DNA replication and the adaptive value of chromatin diminution in copepods.

Chromatin diminution is a precisely controlled, highly repeatable, genome-wide deletion of noncoding heterochromatic segments from the presomatic line. The somatic line is reduced in size and reorganized; the germ line remains unaltered. Little is understood about its mechanistic underpinnings and adaptive significance in the nematodes, copepods, and hagfish in which it occurs. Here, we propose that microcrustacean copepods, whose cytology, development, and evolutionary ecology are well understood from an adaptationist point of view, provide the vehicle to test how chromatin diminution might orchestrate certain cell cycle dynamics, with the consequence of influencing the evolution of nuclear DNA contents, organismal development rates, and body size.

Coincidence, coevolution, or causation? DNA content, cell size, and the C-value enigma.

Variation in DNA content has been largely ignored as a factor in evolution, particularly following the advent of sequence-based approaches to genomic analysis. The significant genome size diversity among organisms (more than 200000-fold among eukaryotes) bears no relationship to organismal complexity and both the origins and reasons for the clearly non-random distribution of this variation remain unclear. Several theories have been proposed to explain this 'C-value enigma' (heretofore known as the 'C-value paradox'), each of which can be described as either a mutation pressure' or 'optimal DNA' theory. Mutation pressure theories consider the large portion of non-coding DNA in eukaryotic genomes as either 'junk' or 'selfish' DNA and are important primarily in considerations of the origin of secondary DNA. Optimal DNA theories differ from mutation pressure theories by emphasizing the strong link between DNA content and cell and nuclear volumes. While mutation pressure theories generally explain this association with cell size as coincidental, the nucleoskeletal theory proposes a coevolutionary interaction between nuclear and cell volume, with DNA content adjusted adaptively following shifts in cell size. Each of these approaches to the C-value enigma is problematic for a variety of reasons and the preponderance of the available evidence instead favours the nucleotypic theory which postulates a causal link between bulk DNA amount and cell volume. Under this view, variation in DNA content is under direct selection via its impacts on cellular and organismal parameters. Until now, no satisfactory mechanism has been presented to explain this nucleotypic effect. However, recent advances in the study of cell cycle regulation suggest a possible 'gene nucleus interaction model' which may account for it. The present article provides a detailed review of the debate surrounding the C-value enigma, the various theories proposed to explain it, and the evidence in favour of a causal connection between DNA content and cell size. In addition, a new model of nucleotypic influence is developed, along with suggestions for further empirical investigation. Finally, some evolutionary implications of genome size diversity are considered, and a broadening of the traditional 'biological hierarchy' is recommended.

The bigger the C-value, the larger the cell: genome size and red blood cell size in vertebrates.

Vertebrate genome sizes vary roughly 350-fold and correlate with a variety of cellular and organismal parameters. Most notable among these is the relationship between genome size ("C-value") and red blood cell (RBC) size, which can be identified within and among each of the five vertebrate classes. This relationship, in turn, leads to important associations between genome size and features such as metabolic rate (at least in homeotherms). The present article describes the correlation between genome size and RBC size in vertebrates and discusses some of the cytological, physiological, and evolutionary implications of this relationship.

Nucleotypic effects without nuclei: genome size and erythrocyte size in mammals.

Previously reported haploid genome sizes (C-values) and erythrocyte sizes (measured as mean dry diameters) were compared for 67 species of mammals representing 31 families and 16 orders. Measurements on erythrocytes of four species of bats were also included in the study. Erythrocyte size was significantly positively correlated with genome size at each of the specific, generic, familial, and ordinal levels, with the relationship becoming much stronger following the exclusion of the order Artiodactyla, a group unique among mammals in terms of red blood cell morphology. Physiologically, these results are relevant in light of the known relationship between C-value and mass-corrected metabolic rate in homeotherms. In evolutionary terms, they provide insights into the constraints on genome expansion among mammals and are therefore of interest in attempts to solve the long-standing C-value enigma (also known as the C-value paradox).

Evolutionary implications of the relationship between genome size and body size in flatworms and copepods.

Genome and body sizes were measured in 38 species of turbellarian flatworms and 16 species of copepod crustaceans. Significant positive relationships existed between genome size and body size in both groups. The slopes of these regressions indicated that increases in cell volume are reinforced by increased cell numbers, or that cell volumes show positive allometric variation with genome size. Genome sizes appear to vary in a discontinuous fashion among congeneric species in both groups, indicating that such changes have occurred rapidly, and with potentially profound effects on important morphological characters.

The effects of chronic plasma cortisol elevation on the feeding behaviour, growth, competitive ability, and swimming performance of juvenile rainbow trout.

Plasma cortisol elevation, a common consequence of stress, occurs in salmonids of subordinate rank; these fish acquire a smaller share of available food and grow more slowly. This study examined the role of cortisol itself in these phenomena. Cortisol implants, with parallel sham and control treatments, were used to create a chronic threefold elevation in plasma cortisol levels in juvenile rainbow trout, and the individual feeding patterns of the fish were evaluated using X-ray radiography. The three treatment groups were (1) held alone and fed to satiation, thereby providing a measure of voluntary appetite, or mixed together in equal proportions and fed to either (2) satiation or (3) half-satiation, thereby allowing assessment of the additional effects of competitive interaction and food limitation. Chronic plasma cortisol elevation had significant negative effects on individual appetite, growth rate, condition factor, and food conversion efficiency, independent of whether the fish were held under unmixed or mixed conditions. Under the latter, mean share of meal was reduced and fin damage increased in cortisol-treated fish; negative growth effects were more severe with food limitation, but the response patterns were otherwise unchanged. Even in the absence of other groups, cortisol-treated fish showed more variable feeding patterns. When compared at the same individual ration levels, cortisol-treated fish had lower growth rates, reflecting a higher "cost of living." Cortisol treatment had no effect on aerobic swimming performance. These results suggest that the structure of the feeding hierarchy may not be determined solely by competitive ability but may also be greatly influenced by differences in the feeding behaviour of unstressed fish versus stressed fish caused by cortisol elevation in the latter.

The modulation of DNA content: proximate causes and ultimate consequences.

The forces responsible for modulating the large-scale features of the genome remain one of the most difficult issues confronting evolutionary biology. Although diversity in chromosomal architecture, nucleotide composition, and genome size has been well documented, there is little understanding of either the evolutionary origins or impact of much of this variation. The 80,000-fold divergence in genome sizes among eukaryotes represents perhaps the greatest challenge for genomic holists. Although some researchers continue to characterize much variation in genome size as a mere by-product of an intragenomic selfish DNA "free-for-all" there is increasing evidence for the primacy of selection in molding genome sizes via impacts on cell size and division rates. Moreover, processes inducing quantum or doubling series variation in gametic or somatic genome sizes are common. These abrupt shifts have broad effects on phenotypic attributes at both cellular and organismal levels and may play an important role in explaining episodes of rapid-or even saltational-character state evolution.