Evolution of the Mitochondrial Cytochrome Oxidase II Gene in Collembola

The sequence of the mitochondrial COII gene has been widely used to estimate phylogenetic relationships at different taxomonic levels across insects. We investigated the molecular evolution of the COII gene and its usefulness for reconstructing phylogenetic relationships within and among four collembolan families. The collembolan COII gene showed the lowest A + T content of all insects so far examined, confirming that the well-known A + T bias in insect mitochondrial genes tends to increase from the basal to apical orders. Fifty-seven percent of all nucleotide positions were variable and most of the third codon positions appeared free to vary. Values of genetic distance between congeneric species and between families were remarkably high; in some cases the latter were higher than divergence values between other orders of insects. The remarkably high divergence levels observed here provide evidence that collembolan taxa are quite old; divergence levels among collembolan families equaled or exceeded divergences among pterygote insect orders. Once the saturated third-codon positions (which violated stationarity of base frequencies) were removed, the COII sequences contained phylogenetic information, but the extent of that information was overestimated by parsimony methods relative to likelihood methods. In the phylogenetic analysis, consistent statistical support was obtained for the monophyly of all four genera examined, but relationships among genera/families were not well supported. Within the genus Orchesella, relationships were well resolved and agreed with allozyme data. Within the genus Isotomurus, although three pairs of populations were consistently identified, these appeared to have arisen in a burst of evolution from an earlier ancestor. Isotomurus italicus always appeared as basal and I. palustris appeared to harbor a cryptic species, corroborating allozyme data. Received: 12 January 1996 / Accepted: 10 August 1996     

基因组序列8-mer频次使用规律及与物种进化的关系

基因组序列k-mer的非随机使用规律及包含的生物学意义一直是人们关注的问题,目前还没有根本性进展。本文以七个物种的全部基因序列为样本,得到各物种基因组序列的8-mer频谱分布。发现狗和牛的频谱有三个峰,而斑马鱼、青鳉鱼、秀丽线虫和酿酒酵母的频谱只有一个峰,鸡的频谱分布形状介于两者之间。将8-mer集合按照XY二核苷含量分类,结果显示只有CG二核苷分类下0CG、1CG和2CG三类子集的频谱形成各自独立的单峰分布。对照随机序列,发现0CG模体是随机进化的,1CG和2CG模体是定向进化的,它们的使用频次远小于随机频次,且这种独立进化分离规律具有物种普适性。三个CG子集频谱之间的距离是产生单峰或多峰现象的根本原因。将七个物种基因组序列标准化到109bp,比较发现1CG和2CG子集频谱与物种进化显著相关,0CG子集频谱与物种进化无显著关系。可以认为三种CG模体各自执行着不同的生物学功能。基因组序列8-mer的独立分离规律为揭示基因组结构、基因组进化以及模体的生物功能提供了一种新的思维方式。     

Molecular evolution of satellite DNA CLsat in lizards of the Darevskia species (Sauria: Lacertidae): correlation with species diversity

The structure and evolution of a satellite DNA family was examined in lizards from the genus Darevskia (family Lacertidae). Comparison of tandem units of repeated DNA (satDNA), CLsat, in all species from the genus Darevskia has shown that their variability is largely based on single-nucleotide substitutions, which constitute about 50 diagnostic positions underlying classification of the family into three subfamilies. Maximum differences between the subfamilies reached 25%. At this level of tandem unit divergence between the subfamilies, no cross-hybridization between them was observed (at 65 degrees C). The individual variability of one subfamily within the species was on average 5% while the variability between species consensuses within a subfamily was 10%. The presence of highly conserved regions in all monomers and some features of their organization show that satellites of all Darevskia species belong to one satDNA family. The organization of unit sequences of satellites CLsat and Agi also detected by us in another lizard genus, Lacerts s. str. was compared. Similarity that was found between these satellites suggests their relatedness and common origin. A possible pathway of evolution of these two satDNA families is proposed. The distribution and content of CLsat repeat subfamilies in all species of the genus was examined by Southern blotting hybridization. Seven species had mainly CLsatI (83 to 96%); three species, approximately equal amounts of CLsatI and CLsatIII (the admixture of CLsatII was 2-3%); and five species, a combination of all three subfamilies in highly varying proportions. Based on these results as well as on zoogeographic views on phylogeny and taxonomy of the Darevskia species, hypotheses on the evolution of molecular-genetic relationships within this genus are advanced.     

The relationship between paternal age, sex ratios, and aneuploidy frequencies in human sperm, as assessed by multicolor FISH.

We studied the frequencies of X- and Y-chromosome-bearing sperm, diploidy and disomy for chromosomes 1, 12, X, and Y in sperm from 10 normal men aged 21-52 years, to determine whether there was any relationship between donor age and any of these variables. Multicolor FISH was used to control for lack of probe hybridization and to distinguish diploid sperm from disomic sperm. A minimum of 10,000 sperm per donor was evaluated for each chromosome, for a total of 225,846 sperm studied. Sperm were considered disomic if two fluorescent signals were separated by a minimal distance of one signal domain. The mean frequencies of X- and Y-bearing sperm were 50.1% and 49.0%, respectively; not significantly different from 50%. There was no correlation between paternal age and "sex ratio" in sperm. Similarly, there was no association between the frequency of diploid sperm (mean, .16%; range, .06-.42%) and donor age. For disomy frequencies, there was no relationship between donor age and disomy 12 (mean, .16%; range, .10%-.25%), XX (mean, .07%; range, .03%-.17%), and XY sperm (mean, .16%; range, .08%-.24%). There was a significant increase in the frequency of YY sperm (P = .04; mean, .18%; range, .10%-.43%) and disomy 1 sperm (P = .01; mean, .11%; range, .05%-.18%) with donor age. In summary, our results do not support a correlation between paternal age and sex ratio or diploidy.     

Endosymbiont phylogenesis in the dryophthoridae weevils: evidence for bacterial replacement

Intracellular symbiosis is widespread in the insect world where it plays an important role in evolution and adaptation. The weevil family Dryophthoridae (Curculionoidea) is of particular interest in intracellular symbiosis evolution with regard to the great economical and ecological features of these invasive insects, and the potential for comparative studies across a wide range of host plants and environments. Here, we have analyzed the intracellular symbiotic bacteria of 19 Dryophthoridae species collected worldwide, representing a wide range of plant species and tissues. All except one (Sitophilus linearis) harbor symbiotic bacteria within specialized cells (the bacteriocytes) assembled as an organ, the bacteriome. Phylogenetic analysis of the 16S rDNA gene sequence of the Dryophthoridae endosymbionts revealed three endosymbiotic clades belonging to gamma3-Proteobacteria and characterized by different GC contents and evolutionary rate. The genus name Candidatus Nardonella was proposed for the ancestral clade infesting Dryophthoridae 100 MYA and represented by five of nine bacterial genera studied. For this clade showing low GC content (40.5% GC) and high evolutionary rate (0.128 substitutions/site per 100 Myr), a single infection and subsequent cospeciation of the host and the endosymbionts was observed. In the two other insect lineage endosymbionts, with relatively high GC content (53.4% and 53.8% GC), competition with ancestral pathogenic bacteria might have occurred, leading to endosymbiont replacement in present-day last insects.     

Molecular Evolution of Satellite DNA CLsat in Lizards from the Genus Darevskia (Sauria: Lacertidae): Correlation with Species Diversity

The structure and evolution of a satellite DNA family was examined in lizards from the genus Darevskia(family Lacertidae). Comparison of tandem units of repeated DNA (satDNA), CLsat, in all species from the genus Darevskiahas shown that their variability is largely explained by single-nucleotide substitutions, which form about 50 diagnostic positions underlying classification of the family into three subfamilies. Maximum differences between the subfamilies reached 25%. At this level of tandem unit divergence in the subfamilies, no cross-hybridization between them was observed (at 65°C). The individual variability within one subfamily within the species was on average 5% while the variability between species consensuses within a subfamily was 10%. The presence of highly conserved regions in all monomers and some features of their organization show that satellites of all Darevskia species belong to one satDNA family. The organization of unit sequences of satellites CLsat and Agi160 also detected by us in another lizard genus, Lacerta s. str. was compared. Similarity that was found between these satellites suggests their relatedness and common origin. A possible pathway of evolution of these two satDNA families is proposed. The distribution and content of CLsat repeat subfamilies in all species of the genus was examined by Southern hybridization. Seven species had mainly CLsatI (83 to 96%); three species, approximately equal amounts of CLsatI and CLsatIII (the admixture of CLsatII was 2–5%); and five species, a combination of all three subfamilies in highly varying proportions. Based on these results as well as on zoogeographic views on the taxonomy and phylogeny of theDarevskia species, hypotheses on the evolution of molecular-genetic relationships within this genus are advanced.     

POLLINATORS DISCRIMINATE AMONG FLORAL HEIGHTS OF A SEXUALLY DECEPTIVE ORCHID: IMPLICATIONS FOR SELECTION

Pollinators have influenced the evolution of many morphological floral traits, although few studies have shown that pollinators have influenced plant height. Chiloglottis trilabra is one of many Australian orchids that deceive and attract male pollinators by mimicking the sex pheromones and morphology of females insects. Orchids in this genus have unusually short flowers whose peduncle elongates dramatically after pollination to approximately twice the original height. In a series of choice experiments in the field, we show that pollinators of C. trilabra strongly discriminate among floral heights, preferring flowers presented at 15 cm-20 cm over flowers presented at lower and higher positions (ranging from 2 cm-100 cm). Our results suggested pollinators have the potential to mediate stabilizing selection for floral height when pollination is limiting. However, the natural height range of the orchid (mean = 10 cm, range 5 cm–15 cm) was lower than the experimentally determined optimum for visitation frequency. This difference may indicate that pollinator-mediated selection does not occur in this species, perhaps because seed set is not sufficiently limited. Alternatively, other life-history factors may counteract pollinator-mediated selection, yielding an evolutionary compromise in height.     

Aggregation behaviour and interspecific relationships in Dermaptera

The rate of gregariousness and intra- and inter-specific attractivity were evaluated under laboratory conditions in five species of Dermaptera. The three species of the genus Forficula (Forficula auricularia L., F. decipiens Gené and F. pubescens Gené) showed distinctly gregarious behaviour in old larvae and adults in the pre-reproductive phase. This gregariousness was seen at the intraspecific level and also among individuals of different species. The other two species, Labidura riparia Pallas and Euborellia moesta Gené, were randomly distributed over the available shelters, with no distinct tendency for aggregation or isolation. In individual tests, insects of all five species were attracted by shelters previously visited by conspecifics in preference to control shelters. For the two non-gregarious species, this phenomenon might function as a burrow marker. The same tests showed an interattractivity for four of the species studied. Only L. riparia did not respond to the trails of the four other species, although its trails were attractive to the other species. The pheromonal nature of the substances involved in the interspecific relationships and the involvement of behaviour in the biology and speciation of Dermaptera are discussed.     

GC-biased gene conversion and selection affect GC content in the Oryza genus (rice)

Base composition varies among and within eukaryote genomes. Although mutational bias and selection have initially been invoked, more recently GC-biased gene conversion (gBGC) has been proposed to play a central role in shaping nucleotide landscapes, especially in yeast, mammals, and birds. gBGC is a kind of meiotic drive in favor of G and C alleles, associated with recombination. Previous studies have also suggested that gBGC could be at work in grass genomes. However, these studies were carried on third codon positions that can undergo selection on codon usage. As most preferred codons end in G or C in grasses, gBGC and selection can be confounded. Here we investigated further the forces that might drive GC content evolution in the rice genus using both coding and noncoding sequences. We found that recombination rates correlate positively with equilibrium GC content and that selfing species (Oryza sativa and O. glaberrima) have significantly lower equilibrium GC content compared with more outcrossing species. As recombination is less efficient in selfing species, these results suggest that recombination drives GC content. We also detected a positive relationship between expression levels and GC content in third codon positions, suggesting that selection favors codons ending with G or C bases. However, the correlation between GC content and recombination cannot be explained by selection on codon usage alone as it was also observed in noncoding positions. Finally, analyses of polymorphism data ruled out the hypothesis that genomic variation in GC content is due to mutational processes. Our results suggest that both gBGC and selection on codon usage affect GC content in the Oryza genus and likely in other grass species.     

Changes in base composition bias of nuclear and mitochondrial genes in lice (Insecta: Psocodea)

While it is well known that changes in the general processes of molecular evolution have occurred on a variety of timescales, the mechanisms underlying these changes are less well understood. Parasitic lice (“Phthiraptera”) and their close relatives (infraorder Nanopsocetae of the insect order Psocodea) are a group of insects well known for their unusual features of molecular evolution. We examined changes in base composition across parasitic lice and bark lice. We identified substantial differences in percent GC content between the clade comprising parasitic lice plus closely related bark lice (=Nanopsocetae) versus all other bark lice. These changes occurred for both nuclear and mitochondrial protein coding and ribosomal RNA genes, often in the same direction. To evaluate whether correlations in base composition change also occurred within lineages, we used phylogenetically controlled comparisons, and in this case few significant correlations were identified. Examining more constrained sites (first/second codon positions and rRNA) revealed that, in comparison to the other bark lice, the GC content of parasitic lice and close relatives tended towards 50 % either up from less than 50 % GC or down from greater than 50 % GC. In contrast, less constrained sites (third codon positions) in both nuclear and mitochondrial genes showed less of a consistent change of base composition in parasitic lice and very close relatives. We conclude that relaxed selection on this group of insects is a potential explanation of the change in base composition for both mitochondrial and nuclear genes, which could lead to nucleotide frequencies closer to random expectation (i.e., 50 % GC) in the absence of any mutation bias. Evidence suggests this relaxed selection arose once in the non-parasitic common ancestor of Phthiraptera + Nanopsocetae and is not directly related to the evolution of the parasitism in lice.     

Phylogenetic and geographic variation in host breadth and composition by herbivorous amphipods in the family Ampithoidae

Predicting the host range for herbivores has been a major aim of research into plant-herbivore interactions and an important model system for understanding the evolution of feeding specialization. Among many terrestrial insects, host range is strongly affected by herbivore phylogeny and long historical associations between particular herbivore and plant taxa. For small herbivores in marine environments, it is known that the evolution of host use is sculpted by several ecological factors (e.g., food quality, value as a refuge from predators, and abiotic forces), but the potential for phylogenetic constraints on host use remains largely unexplored. Here, we analyze reports of host use of herbivorous amphipods from the family Ampithoidae (102 amphipod species from 12 genera) to test the hypotheses that host breadth and composition vary among herbivore lineages, and to quantify the extent to which nonpolar secondary metabolites mediate these patterns. The family as a whole, and most individual species, are found on a wide variety of macroalgae and seagrasses. Despite this polyphagous host use, amphipod genera consistently differed in host range and composition. As an example, the genus Peramphithoe rarely use available macrophytes in the order Dictyotales (e.g., Dictyota) and as a consequence, display a more restricted host range than do other genera (e.g., Ampithoe, Cymadusa, or Exampithoe). The strong phylogenetic effect on host use was independent of the uneven distribution of host taxa among geographic regions. Algae that produced nonpolar secondary metabolites were colonized by higher numbers of amphipod species relative to chemically poor genera, consistent with the notion that secondary metabolites do not provide algae an escape from amphipod herbivory. In contrast to patterns described for some groups of phytophagous insects, marine amphipods that use chemically rich algae tended to have broader, not narrower, host ranges. This result suggests that an evolutionary advantage to metabolite tolerance in marine amphipods may be that it increases the availability of appropriate algal hosts (i.e., enlarges the resource base).     

The evolution of climate tolerance in conifer‐feeding aphids in relation to their host's climatic niche

Climate adaptation has major consequences in the evolution and ecology of all living organisms. Though phytophagous insects are an important component of Earth's biodiversity, there are few studies investigating the evolution of their climatic preferences. This lack of research is probably because their evolutionary ecology is thought to be primarily driven by their interactions with their host plants. Here, we use a robust phylogenetic framework and species‐level distribution data for the conifer‐feeding aphid genus Cinara to investigate the role of climatic adaptation in the diversity and distribution patterns of these host‐specialized insects. Insect climate niches were reconstructed at a macroevolutionary scale, highlighting that climate niche tolerance is evolutionarily labile, with closely related species exhibiting strong climatic disparities. This result may suggest repeated climate niche differentiation during the evolutionary diversification of Cinara. Alternatively, it may merely reflect the use of host plants that occur in disparate climatic zones, and thus, in reality the aphid species' fundamental climate niches may actually be similar but broad. Comparisons of the aphids' current climate niches with those of their hosts show that most Cinara species occupy the full range of the climatic tolerance exhibited by their set of host plants, corroborating the hypothesis that the observed disparity in Cinara species' climate niches can simply mirror that of their hosts. However, 29% of the studied species only occupy a subset of their hosts' climatic zone, suggesting that some aphid species do indeed have their own climatic limitations. Our results suggest that in host‐specialized phytophagous insects, host associations cannot always adequately describe insect niches and abiotic factors must be taken into account.     

Paternal signature in kin recognition cues of a social insect: concealed in juveniles,revealed in adults

Kin recognition is a key mechanism to direct social behaviours towards related individuals or avoid inbreeding depression. In insects, recognition is generally mediated by cuticular hydrocarbon (CHC) compounds, which are partly inherited from parents. However, in social insects, potential nepotistic conflicts between group members from different patrilines are predicted to select against the expression of patriline-specific signatures in CHC profiles. Whereas this key prediction in the evolution of insect signalling received empirical support in eusocial insects, it remains unclear whether it can be generalized beyond eusociality to less-derived forms of social life. Here, we addressed this issue by manipulating the number of fathers siring clutches tended by females of the European earwig, Forficula auricularia, analysing the CHC profiles of the resulting juvenile and adult offspring, and using discriminant analysis to estimate the information content of CHC with respect to the maternal and paternal origin of individuals. As predicted, if paternally inherited cues are concealed during family life, increases in mating number had no effect on information content of CHC profiles among earwig juveniles, but significantly decreased the one among adult offspring. We suggest that age-dependent expression of patriline-specific cues evolved to limit the risks of nepotism as family-living juveniles and favour sibling-mating avoidance as group-living adults. These results highlight the role of parental care and social life in the evolution of chemical communication and recognition cues.     

High mating frequency and variation with lineage ratio in dependent-lineage harvester ants

Explaining the evolution of multiple mating is a challenge because of the associated costs. For social insects, mating frequency may have fitness consequences due to effects on social interactions or genetic diversity within colonies. Here, we investigated the evolution of mating frequency in a social insect species with a unique genetic system that requires multiple mating. In certain populations of Pogonomyrmex harvester ants, there are two interbreeding yet genetically distinct mitochondrial lineages. Queens must mate with males of the opposite lineage to produce workers and with males of the same lineage to produce reproductive females. We expected queens of the dependent-lineage system to exhibit high mating frequencies relative to other social insects. Furthermore, we expected queens from populations of highly asymmetric lineage ratios to exhibit even higher mating frequencies, to adequately sample the population and successfully mate with males of the less common lineage. To test these predictions, we estimated the mating frequency of 16 P. barbatus queens, and compared these mating frequencies between two populations, one with relatively equal lineage ratio (60:40) and a second with a highly asymmetrical lineage ratio (96:4). Overall, observed mating frequency exceeded 10, which is high in comparison to other social insects, and our estimates of effective mating frequency were among the highest of Pogonomyrmex species. Mating frequency at the site with the asymmetrical lineage ratio was also significantly higher than the site with the more even ratio. Our results suggest that obligate multiple mating as well as lineage ratio contribute to the evolution of high mating frequency in dependent-lineage populations.     

IS SPECIALIZATION A DEAD END? THE PHYLOGENY OF HOST USE IN DENDROCTONUS BARK BEETLES (SCOLYTIDAE)

Ecological explanations for the prevalence of resource specialists are abundant, whereas phylogenetic evidence on their origins is scarce. In this paper, we provide a molecular phylogenetic study of the 19 specialist or generalist species in the bark beetle genus Dendroctonus, which collectively attack species in four different genera in the conifer family Pinaceae. Given substantial variation in diet breadth, we asked two general questions concerning the evolution of resource use in this group. How conservative is the evolution of host use in these insects? Does specialization tend to be derived (i.e., a “dead end”)? To answer these questions, we estimated the phylogeny of Dendroctonus using mitochondrial DNA sequences and mapped transitions in resource use on the resulting phylogeny estimate. The evolution of affiliations with Pinus and Picea hosts in Dendroctonus was conservative among beetle species (PTP test; P < 0.012), but there was no significant correspondence between the phylogeny of these beetles and the phylogeny among their Pinaceae hosts (among genera, P = 0.28; among Pinus species, P = 0.82). Degree of specialization, as measured in the proportion of hosts used, was bimodally distributed with “generalist” species utilizing < 60% of the congeneric hosts within their range and six specialist species utilizing < 40% of the available hosts. Among the generalists, we found a strong correlation between the number of hosts encountered and the number of hosts utilized (R = 0.97, P < 0.0001), whereas there was no significant correlation among the specialists (R = 0.27, P = 0.59). The evolution of specialization in Dendroctonus proved highly labile—specialists arose from generalists at least six separate times (without reversal) all in derived positions, and closer examination of some specialists revealed instances where they appear to have lost particular host species from their diet. However, evidence from the ecological literature also suggests that several Dendroctonus generalists may have increased their range of host genera within the Pinaceae.     

Application of flow cytometry for exploring the evolution of Geosmithia fungi living in association with bark beetles: the role of conidial DNA content

Geosmithia belongs among fungi living in symbiosis with phloem-feeding bark beetles. Several species have altered their ecology to that of obligatory symbiosis with ambrosia beetles, which has led to a shift in their phenotype and caused formation of large spherical conidia. In this study, we pose the following questions; (1) Is the conidial DNA content of Geosmithia correlated with conidial volume?; (2) Is the DNA content of Geosmithia related to the degree of mutual dependence between Geosmithia and their vector? There was a positive and strong correlation between conidial DNA content and conidial volume in Geosmithia. Also species more narrowly associated with the vector tend to have a larger conidial DNA content and volume than less narrowly associated species. Ambrosia fungi achieved the biggest conidial DNA content and volume compared to other species. We suppose that polyploidisation occurred during the evolution of ambrosia species in the genus Geosmithia.     

Structure of the phytophagous insect fauna of Onopordum thistles in the northern Mediterranean basin

Thistles of the genus Onopordum (Asteraceae: Cardueae) have become serious weed problems in parts of Australia and North America following introduction from their native Eurasian ranges, and are the target of a biological control programme in Australia. This paper analyses the results of three separate surveys of insects feeding and breeding on thistles of the genus Onopordum in the Mediterranean region. Of the 129 species found feeding, 74 species also bred on these thistles. Endophages comprised 54% of the breeding insects, with species that fed in the capitula and within the rosette crown/root area predominating. The Coleoptera and Hemiptera were the dominant endophage and ectophage taxa, respectively. Differences in the Onopordum fauna were noted between host species and between geographic regions, but this was largely due to the generalist component of the fauna. Onopordum specialists showed little variability across regions or between different species within the host genus. On comparing the fauna of Onopordum with that of other thistle genera, it was suggested that plant-herbivore interactions between Onopordum and its fauna are less highly evolved, in that there is less host specialization and a lesser degree of niche partitioning. This appears to be related to the low rate of speciation and habitat specialization within the host genus itself. The potential of certain genus-specific insects as biological control agents for Onopordum spp. is discussed. It is argued that the absence of a high degree of niche specialization could favour successful biological control, as the agent would be capable of inflicting damage over a wider range of resource gradients.     

Muscular connections between the gut and thoracic exoskeleton in some orthopteroid insects

Muscle and connective tissue strands linking the prothoracic cuticle to the posterior foregut and anterior midgut are described for 17 species of orthopteroid insects: Dinocras cephalotes (Plecoptera); Forficula aurkularia (Dermaptera); Acheta domesticus, Gryllus bimaculatus, Brachytrupes sp. and Phaeophyllacris spectrum (Gryllidae); Schizodactylus monstrosus (Schizodactylidae); Hemideina crassidens (Stenopelmatidae); Ephippiger ephippiger, Tettigonia viridissima, Conocephalus dorsalis and Pholidoptera griseoaptera (Tetligoniidae); Schislocerca gregaria, Omocestus viridulus and Chorthippus brunneus (Acrididae), and Carausms morosus (Phasmida). Of the 18 species examined, only the Gryllotalpa sp. (Gryllotalpidae) lacked extrinsic strands to the post-cephalic gut. The arrangement of the strands in the various species is compared, and the possible evolution and function of the strands are briefly discussed.     

Phylogenetic Relationships of American Willows (Salix L., Salicaceae)

Salix L. is the largest genus in the family Salicaceae (450 species). Several classifications have been published, but taxonomic subdivision has been under continuous revision. Our goal is to establish the phylogenetic structure of the genus using molecular data on all American willows, using three DNA markers. This complete phylogeny of American willows allows us to propose a biogeographic framework for the evolution of the genus. Material was obtained for the 122 native and introduced willow species of America. Sequences were obtained from the ITS (ribosomal nuclear DNA) and two plastid regions, matK and rbcL. Phylogenetic analyses (parsimony, maximum likelihood, Bayesian inference) were performed on the data. Geographic distribution was mapped onto the tree. The species tree provides strong support for a division of the genus into two subgenera, Salix and Vetrix. Subgenus Salix comprises temperate species from the Americas and Asia, and their disjunction may result from Tertiary events. Subgenus Vetrix is composed of boreo-arctic species of the Northern Hemisphere and their radiation may coincide with the Quaternary glaciations. Sixteen species have ambiguous positions; genetic diversity is lower in subg. Vetrix. A molecular phylogeny of all species of American willows has been inferred. It needs to be tested and further resolved using other molecular data. Nonetheless, the genus clearly has two clades that have distinct biogeographic patterns.     

Spontaneous frequencies of aneuploid and diploid sperm in 10 normal Chinese men: assessed by multicolor fluorescence in situ hybridization

Many studies have been published establishing the background frequencies of disomic and diploid sperm in normal men by fluorescence in situ hybridization (FISH) analysis, with highly significant variance among the reports. Besides interdonor heterogeneity and differences in the experimental protocols used, the question of inherent differences in chromosome malsegregation and meiotic arrest among different geographic and ethnic groups of donors has been raised. In this study, multicolor FISH analysis was carried out on semen samples from 10 nonsmoking, nondrinking Chinese men from the People's Republic of China. The results were compared to FISH data on 10 nonsmoking, nondrinking Canadians under the same experimental conditions, in the same laboratory. A total of 200,497 sperm was scored in the Chinese donors and compared to 202,320 sperm from Canadian donors. Approximately 10,000 sperm per chromosome probe per donor were analyzed. The mean hybridization efficiency was 99.99%. The frequencies of X-bearing and Y-bearing sperm were not significantly different from the expected 50% for each individual and for the combined data from all donors (49.73% vs. 49.46%, P = 0.3946). The mean disomy frequencies (range) were 0.07% (0.02%-0.12%) for chromosome 13, 0.18% (0.09%-0.19%) for chromosome 21, 0.05% (0. 01%-0.09%) for 24,XX, 0.02% (0.01%-0.06%) for 24,YY, and 0.29% (0. 13%-0.49%) for 24,XY. The mean diploidy frequency (range) was 0.38% (0.22%-0.73%) for 13-21 hybridizations and 0.32% (0.07%-0.70%) for XY hybridizations. Highly significant interdonor heterogeneity was found for diploidy (P = 0.0000) and for XY disomy (P = 0.0011), but no age effect was observed in any category of disomic or diploid sperm. The data reported here show no marked differences in disomy and diploidy frequencies between the mainland Chinese and Canadian groups, if donor heterogeneity is taken into account.