3D shape analysis of grass silica short cell phytoliths: a new method for fossil classification and analysis of shape evolution.

Fossil grass silica short cell phytoliths (GSSCP) have been used to reconstruct the biogeography of Poaceae, untangle crop domestication history and detect past vegetation shifts. These inferences depend on accurately identifying the clade to which the fossils belong. Patterns of GSSCP shape and size variation across the family have not been established and current classification methods are subjective or based on a 2D view that ignores important 3D shape variation. Focusing on Poaceae subfamilies Anomochlooideae, Pharoideae, Pueliodieae, Bambusoideae and Oryzoideae, we observed in situ GSSCP to establish their orientation and imaged isolated GSSCP using confocal microscopy to produce 3D models. 3D geometric morphometrics was used to analyze GSSCP shape and size. Classification models were applied to GSSCP from Eocene sediments from Nebraska, USA, and Anatolia, Turkey. There were significant shape differences between nearly all recognized GSSCP morphotypes and between clades with shared morphotypes. Most of the Eocene GSSCP were classified as woody bamboos with some distinctive Nebraska GSSCP classified as herbaceous bamboos. 3D morphometrics hold great promise for GSSCP classification. It accounts for the complete GSSCP shape, automates size measurements and accommodates the complete range of morphotypes within a single analytical framework.

Phylogenetic relationships within Parianinae (Poaceae: Bambusoideae: Olyreae) with emphasis on Eremitis: Evidence from nuclear and plastid DNA sequences, macromorphology, and pollen ectexine patterns.

Eremitis, Pariana, and Parianella are herbaceous bamboos (tribe Olyreae) included in the subtribe Parianinae, which is characterized by the presence of fimbriae at the apex of the leaf sheaths and exclusively spiciform synflorescences. We analyzed 43 samples of herbaceous and woody bamboos in order to infer relationships within the Parianinae, based on combined data from the nuclear ribosomal internal transcribed spacer (ITS) and plastid DNA (rpl32-trnL and trnD-trnT spacers). Bayesian inference, maximum likelihood, and maximum parsimony methods were applied, and macro- and micromorphological aspects were also analyzed, including the ectexine patterns of pollen grains. Parianinae is represented by three well-supported lineages in our analyses: (1) Parianella, endemic to southern Bahia, Brazil; (2) Pariana sensu stricto with a broad distribution in southern Central America and northern South America, especially in the Amazon region; and (3) Eremitis, endemic to the Brazilian Atlantic Forest, from the states of Pernambuco to Rio de Janeiro, including one species previously described as a member of Pariana. Our molecular phylogeny showed that Pariana, as historically circumscribed, is not monophyletic, by recovering Pariana sensu stricto as strongly supported and sister to Eremitis + Pariana multiflora, with Parianella sister to the Pariana-Eremitis clade. Morphological features of their synflorescences and differences in ectexine patterns characterize each lineage. Based on all these characters and the phylogenetic results, Pariana multiflora, endemic to the state of Espírito Santo, Brazil, is transferred to Eremitis.

Fusoid cells in the grass family Poaceae (Poales): a developmental study reveals homologies and suggests new insights into their functional role in young leaves.

Background and Aims: In mature grass leaf blades as seen in cross-section, oblong cell-like structures have been interpreted most recently as intercellular gas spaces delimited by successive collapsed fusoid cells. These cells have been reported in at least seven of 12 subfamilies of Poaceae and are considered a synapomorphy for the family; however, no developmental work has been performed to verify their meristematic origin or to assess possible homologies within the graminid clade (= Flagellariaceae + [(Joinvilleaceae + Ecdeiocoleaceae) + Poaceae]) or among subfamilies of Poaceae. A developmental study was therefore carried out, including 20 species in three families (Flagellariaceae, Joinvilleaceae and Poaceae), representing the earlier-diverging and derived branches within the graminid clade and Poaceae. Methods: Light microscopy was combined with scanning electron microscopy, cryoscanning electron microscopy and transmission electron microscopy to study the development of leaves taken from the shoot apex of young plants. Mature leaf blades also were taken from living or dried plants and the mid-portion was studied. Key Results: Developmental results show that, in mature leaf blades as seen in cross-section, one apparent fusoid cell is typically a cavity resulting from the collapse of the initial fusoid cell and its internal divisions, which are herein interpreted as derivative cells with formation of cell plates only. Each cavity is delimited by successive collapsed fusoid cells arranged perpendicularly to the veins. Fusoid cells in all studied Poaceae members originate from the ground meristem, as do the colourless cells in Joinvillea ascendens (Joinvilleaceae). These two types of mesophyll cell have a strongly similar ontogeny, distinguished mainly by the collapse of the fusoid cells in Poaceae, which is not observed in the colourless cells in J. ascendens. Conclusions: Within the Poaceae, the meristematic origin of fusoid cells is the same in the early-diverging lineages, BOP clade and Panicoideae, and thus they are homologous within the family. The same topography and meristematic origin suggest that fusoid cells in Poaceae and colourless cells in Joinvilleaceae are homologous. The results also suggest that the role played by the fusoid cells in young grass leaves is related to synthesis and storage of starch granules at early stages of development.

Evolutionary relationships in Panicoid grasses based on plastome phylogenomics (Panicoideae; Poaceae).

BACKGROUND: Panicoideae are the second largest subfamily in Poaceae (grass family), with 212 genera and approximately 3316 species. Previous studies have begun to reveal relationships within the subfamily, but largely lack resolution and/or robust support for certain tribal and subtribal groups. This study aims to resolve these relationships, as well as characterize a putative mitochondrial insert in one linage. RESULTS: 35 newly sequenced Panicoideae plastomes were combined in a phylogenomic study with 37 other species: 15 Panicoideae and 22 from outgroups. A robust Panicoideae topology largely congruent with previous studies was obtained, but with some incongruences with previously reported subtribal relationships. A mitochondrial DNA (mtDNA) to plastid DNA (ptDNA) transfer was discovered in the Paspalum lineage. CONCLUSIONS: The phylogenomic analysis returned a topology that largely supports previous studies. Five previously recognized subtribes appear on the topology to be non-monophyletic. Additionally, evidence for mtDNA to ptDNA transfer was identified in both Paspalum fimbriatum and P. dilatatum, and suggests a single rare event that took place in a common progenitor. Finally, the framework from this study can guide larger whole plastome sampling to discern the relationships in Cyperochloeae, Steyermarkochloeae, Gynerieae, and other incertae sedis taxa that are weakly supported or unresolved.

Phylogenetic estimation and morphological evolution of Arundinarieae (Bambusoideae: Poaceae) based on plastome phylogenomic analysis.

We explored phylogenetic relationships among the twelve lineages of the temperate woody bamboo clade (tribe Arundinarieae) based on plastid genome (plastome) sequence data. A representative sample of 28 taxa was used and maximum parsimony, maximum likelihood and Bayesian inference analyses were conducted to estimate the Arundinarieae phylogeny. All the previously recognized clades of Arundinarieae were supported, with Ampelocalamus calcareus (Clade XI) as sister to the rest of the temperate woody bamboos. Well supported sister relationships between Bergbambos tessellata (Clade I) and Thamnocalamus spathiflorus (Clade VII) and between Kuruna (Clade XII) and Chimonocalmus (Clade III) were revealed by the current study. The plastome topology was tested by taxon removal experiments and alternative hypothesis testing and the results supported the current plastome phylogeny as robust. Neighbor-net analyses showed few phylogenetic signal conflicts, but suggested some potentially complex relationships among these taxa. Analyses of morphological character evolution of rhizomes and reproductive structures revealed that pachymorph rhizomes were most likely the ancestral state in Arundinarieae. In contrast leptomorph rhizomes either evolved once with reversions to the pachymorph condition or multiple times in Arundinarieae. Further, pseudospikelets evolved independently at least twice in the Arundinarieae, but the ancestral state is ambiguous.

Evolution of the bamboos (Bambusoideae; Poaceae): a full plastome phylogenomic analysis.

BACKGROUND: Bambusoideae (Poaceae) comprise three distinct and well-supported lineages: tropical woody bamboos (Bambuseae), temperate woody bamboos (Arundinarieae) and herbaceous bamboos (Olyreae). Phylogenetic studies using chloroplast markers have generally supported a sister relationship between Bambuseae and Olyreae. This suggests either at least two origins of the woody bamboo syndrome in this subfamily or its loss in Olyreae. RESULTS: Here a full chloroplast genome (plastome) phylogenomic study is presented using the coding and noncoding regions of 13 complete plastomes from the Bambuseae, eight from Olyreae and 10 from Arundinarieae. Trees generated using full plastome sequences support the previously recovered monophyletic relationship between Bambuseae and Olyreae. In addition to these relationships, several unique plastome features are uncovered including the first mitogenome-to-plastome horizontal gene transfer observed in monocots. CONCLUSIONS: Phylogenomic agreement with previous published phylogenies reinforces the validity of these studies. Additionally, this study presents the first published plastomes from Neotropical woody bamboos and the first full plastome phylogenomic study performed within the herbaceous bamboos. Although the phylogenomic tree presented in this study is largely robust, additional studies using nuclear genes support monophyly in woody bamboos as well as hybridization among previous woody bamboo lineages. The evolutionary history of the Bambusoideae could be further clarified using transcriptomic techniques to increase sampling among nuclear orthologues and investigate the molecular genetics underlying the development of woody and floral tissues.

Resolving deep relationships of PACMAD grasses: a phylogenomic approach.

BACKGROUND: Plastome sequences for 18 species of the PACMAD grasses (subfamilies Panicoideae, Aristidoideae, Chloridoideae, Micrairoideae, Arundinoideae, Danthonioideae) were analyzed phylogenomically. Next generation sequencing methods were used to provide complete plastome sequences for 12 species. Sanger sequencing was performed to determine the plastome of one species, Hakonechloa macra, to provide a reference for annotation. These analyses were conducted to resolve deep subfamilial relationships within the clade. Divergence estimates were assessed to determine potential factors that led to the rapid radiation of this lineage and its dominance of warmer open habitats. RESULTS: New plastomes were completely sequenced and characterized for 13 PACMAD species. An autapomorphic ~1140 bp deletion was found in Hakonechloa macra putatively pseudogenizing rpl14 and eliminating rpl16 from this plastome. Phylogenomic analyses support Panicoideae as the sister group to the ACMAD clade. Complete plastome sequences provide greater support at deep nodes within the PACMAD clade. The initial diversification of PACMAD subfamilies was estimated to occur at 32.4 mya. CONCLUSIONS: Phylogenomic analyses of complete plastomes provides resolution for deep relationships of PACMAD grasses. The divergence estimate of 32.4 mya at the crown node of the PACMAD clade coincides with the Eocene-Oligocene Transition (EOT). The Eocene was a period of global cooling and drying, which led to forest fragmentation and the expansion of open habitats now dominated by these grasses. Understanding how these grasses are related and determining a cause for their rapid radiation allows for future predictions of grassland distribution in the face of a changing global climate.

Independent allopolyploidization events preceded speciation in the temperate and tropical woody bamboos.

The objectives of the current study were to investigate the origin of polyploidy in the woody bamboos and examine putative hybrid relationships in one major lineage (the temperate woody bamboos, tribe Arundinarieae). Phylogenetic analyses were based on sequence data from three nuclear loci and 38 species in 27 genera. We identify six ancestral genome donors for contemporary bamboo lineages: temperate woody bamboos (tribe Arundinarieae) contain genomes A and B, tropical woody bamboos (tribe Bambuseae) contain genomes C and D, and herbaceous bamboos (tribe Olyreae) contain genome H; some hexaploid paleotropical bamboos contain genome E in addition to C and D. Molecular data indicate that allopolyploidy arose independently in temperate (AABB) and tropical woody lineages (CCDD and CCDDEE), and speciation occurred subsequent to polyploidization. Moreover, hybridization has played a surprising and recurrent role in bamboo evolution, generating allohexaploid species in the paleotropical clade and intergeneric hybrids among the allotetraploid temperate bamboos. We suggest this complex history of reticulate evolution is at least partially responsible for the taxonomic difficulty associated with the woody bamboos. This newly-resolved phylogenetic framework reflects a major step forward in our understanding of bamboo biodiversity and has important implications for the interpretation of bamboo phylogenomics.

A molecular phylogeny of Raddia and its allies within the tribe Olyreae (Poaceae, Bambusoideae) based on noncoding plastid and nuclear spacers.

The plastid spacer trnD-trnT and the nuclear ribosomal internal transcribed spacer (ITS) were sequenced for 37 samples of herbaceous bamboos (Poaceae: Olyreae), including all Raddia species and allied genera, as well as two members of the woody bamboos (tribes Bambuseae and Arundinarieae), in order to examine their relationships. The sequences were analyzed using maximum parsimony and Bayesian inference. Both the individual and combined analyses of ITS and trnD-trnT supported Olyreae as a monophyletic group. All species of Raddia also formed a well-supported monophyletic group, and combined datasets allowed us to outline some relationships within this group. Individual analyses indicated incongruence regarding the sister group of Raddia, with ITS data weakly indicating Raddiella malmeana whereas trnD-trnT data supported Sucrea maculata in this position. However, the combined analysis supported Sucrea as sister to Raddia, although the monophyly of Sucrea is not well supported. Parodiolyra is paraphyletic to Raddiella in all analyses; Olyra is also paraphyletic, with species of Lithachne, Arberella and Cryptochloa nested within it. Eremitis and Pariana appeared as an isolated clade within Olyreae, and the position of the New Guinean Buergersiochloa remains uncertain within this tribe.

Biogeography and phylogenomics of New World Bambusoideae (Poaceae), revisited.

UNLABELLED: • PREMISE OF THE STUDY: New World Bambusoideae have only recently been studied in a phylogenomic context. Plastome sequences were determined and analyzed from Arundinaria appalachiana, A. tecta, and Olyra latifolia, to refine our knowledge of their evolution and historical biogeography. A correction is noted regarding an error in an earlier report on the biogeography of Cryptochloa• METHODS: Single-end DNA libraries were prepared and sequenced on the Illumina platform. Complete plastomes were assembled and analyzed with 13 other Poaceae.• KEY RESULTS: Complete sampling in Arundinaria and an additional species of Olyreae gave a more detailed picture of their evolution/historical biogeography. Phylogenomic analyses indicated that the first major divergence in Arundinaria occurred around 2.3 to 3.2 mya and that Arundinaria tecta and A appalachiana diverged from their common ancestor around 0.57 to 0.82 mya. Estimates of the divergence of Olyra latifolia from Cryptochloa strictiflora ranged from 14.6 to 20.7 mya. The age of the stem node of Olyreae ranged from an estimated 26.9 to 38.2 mya.• CONCLUSIONS: Estimates of divergences in Arundinaria can be correlated with paleoclimatic events including an early Pliocene warming, subsequent cooling, and North American glaciations. Discriminating between alternate evolutionary/biogeographic scenarios in Olyreae is challenging.

Genome assemblies of 11 bamboo species highlight diversification induced by dynamic subgenome dominance.

Polyploidy (genome duplication) is a pivotal force in evolution. However, the interactions between parental genomes in a polyploid nucleus, frequently involving subgenome dominance, are poorly understood. Here we showcase analyses of a bamboo system (Poaceae: Bambusoideae) comprising a series of lineages from diploid (herbaceous) to tetraploid and hexaploid (woody), with 11 chromosome-level de novo genome assemblies and 476 transcriptome samples. We find that woody bamboo subgenomes exhibit stunning karyotype stability, with parallel subgenome dominance in the two tetraploid clades and a gradual shift of dominance in the hexaploid clade. Allopolyploidization and subgenome dominance have shaped the evolution of tree-like lignified culms, rapid growth and synchronous flowering characteristic of woody bamboos as large grasses. Our work provides insights into genome dominance in a remarkable polyploid system, including its dependence on genomic context and its ability to switch which subgenomes are dominant over evolutionary time.

A maceration technique for soft plant tissue without hazardous chemicals.

Premise: Current methods for maceration of plant tissue use hazardous chemicals. The new method described here improves the safety of dissection and maceration of soft plant tissues for microscopic imaging by using the harmless enzyme pectinase. Methods and Results: Leaf material from a variety of land plants was obtained from living plants and dried herbarium specimens. Concentrations of aqueous pectinase and soaking schedules were optimized, and tissues were manually dissected while submerged in fresh solution following a soaking period. Most leaves required 2-4 h of soaking; however, delicate leaves could be macerated after 30 min while tougher leaves required 12 h to 3 days of soaking. Staining techniques can also be used with this method, and permanent or semi-permanent slides can be prepared. The epidermis, vascular tissue, and individual cells were imaged at magnifications of 10× to 400×. Only basic safety precautions were needed. Conclusions: This pectinase method is a cost-effective and safe way to obtain images of epidermal peels, separated tissues, or isolated cells from a wide range of plant taxa.

Molecular phylogeny of the arthrostylidioid bamboos (Poaceae: Bambusoideae: Bambuseae: Arthrostylidiinae) and new genus Didymogonyx.

We present the first multi-locus chloroplast phylogeny of Arthrostylidiinae, a subtribe of neotropical woody bamboos. The morphological diversity of Arthrostylidiinae makes its taxonomy difficult and prior molecular analyses of bamboos have lacked breadth of sampling within the subtribe, leaving internal relationships uncertain. We sampled 51 taxa, chosen to span the range of taxonomic diversity and morphology, and analyzed a combined chloroplast DNA dataset with six chloroplast regions: ndhF, trnD-trnT, trnC-rpoB, rps16-trnQ, trnT-trnL, and rpl16. A consensus of maximum parsimony and Bayesian inference analyses reveals monophyly of the Arthrostylidiinae and four moderately supported lineages within it. Six previously recognized genera were monophyletic, three polyphyletic, and two monotypic; Rhipidocladum sect. Didymogonyx is here raised to generic status. When mapped onto our topology, many of the morphological characters show homoplasy.

Intraspeaker Priming across the New Zealand English Short Front Vowel Shift.

A growing body of research in psycholinguistics, corpus linguistics, and sociolinguistics shows that we have a strong tendency to repeat linguistic material that we have recently produced, seen, or heard. The present paper investigates whether priming effects manifest in continuous phonetic variation the way it has been reported in phonological, morphological, and syntactic variation. We analyzed nearly 60,000 tokens of vowels involved in the New Zealand English short front vowel shift (SFVS), a change in progress in which trap/dress move in the opposite direction to kit, from a topic-controlled corpus of monologues (166 speakers), to test for effects that are characteristic of priming phenomena: repetition, decay, and lexical boost. Our analysis found evidence for all three effects. Tokens that were relatively high and front tended to be followed by tokens that were also high and front; the repetition effect weakened with greater time between the prime and target; and the repetition effect was stronger if the prime and target belonged to (different tokens of) the same word. Contrary to our expectations, however, the cross-vowel effects suggest that the repetition effect responded not to the direction of vowel changes within the SFVS, but rather the peripherality of the tokens. We also found an interaction between priming behavior and gender, with stronger repetition effects among men than women. While these findings both indicate that priming manifests in continuous phonetic variation and provide further evidence that priming is among the factors providing structure to intraspeaker variation, they also challenge unitary accounts of priming phenomena.

Incidence, correlates, and interventions used for pressure ulcers of the ear.

Skin prevalence audits revealed annual increases in incidence of pressure ulcers of the ear. A research study was conducted to assess correlates of the problem. Study results guided clinical practice changes that reduced the incidence to zero.

Notes on leaf micromorphology of the rare herbaceous bamboo Buergersiochloa bambusoides Pilg. (Olyreae, Poaceae) from New Guinea and its taxonomic implications.

We present notes on the leaf micromorphology of Buergersiochloa bambusoides, a rare species from New Guinea and included in Buergersiochloinae, one of three subtribes of the herbaceous bamboos (tribe Olyreae). We used scanning electron microscopy and light microscopy to analyze the microcharacters of both adaxial and abaxial leaf surfaces. Within the Olyreae, saddle-shaped silica bodies in both the costal and intercostal zones are considered unique to Buergersiochloinae. Simple, circular and very small papillae are observed on the adaxial surface, and for the first time, branched papillae on the abaxial surface are observed in B. bambusoides. On the abaxial surface, there are papillae on long cells associated with the stomatal complexes. Bicellular microhairs are the only trichomes present and they are found almost exclusively on the abaxial surface. The saddle-shaped silica bodies are the most taxonomically important among the microcharacters observed on the leaf surface of B. bambusoides.

Integrated Genomic Analyses From Low-Depth Sequencing Help Resolve Phylogenetic Incongruence in the Bamboos (Poaceae: Bambusoideae).

The bamboos (Bambusoideae, Poaceae) comprise a major grass lineage with a complex evolutionary history involving ancient hybridization and allopolyploidy. About 1700 described species are classified into three tribes, Olyreae (herbaceous bamboos), Bambuseae (tropical woody bamboos), and Arundinarieae (temperate woody bamboos). Nuclear analyses strongly support monophyly of the woody tribes, whereas plastome analyses strongly support paraphyly, with Bambuseae sister to Olyreae. Our objectives were to clarify the origin(s) of the woody bamboo tribes and resolve the nuclear vs. plastid conflict using genomic tools. For the first time, plastid and nuclear genomic information from the same bamboo species were combined in a single study. We sampled 51 species of bamboos representing the three tribes, estimated their genome sizes and generated low-depth sample sequence data, from which plastomes were assembled and nuclear repeats were analyzed. The distribution of repeat families was found to agree with nuclear gene phylogenies, but also provides novel insights into nuclear evolutionary history. We infer two early, independent hybridization events, one between an Olyreae ancestor and a woody ancestor giving rise to the two Bambuseae lineages, and another between two woody ancestors giving rise to the Arundinarieae. Retention of the Olyreae plastome associated with differential dominance of nuclear genomes and subsequent diploidization in some lineages explains the paraphyly observed in plastome phylogenetic estimations. We confirm ancient hybridization and allopolyploidy in the origins of the extant woody bamboo lineages and propose biased fractionation and diploidization as important factors in their evolution.

The Streptochaeta Genome and the Evolution of the Grasses.

In this work, we sequenced and annotated the genome of Streptochaeta angustifolia, one of two genera in the grass subfamily Anomochlooideae, a lineage sister to all other grasses. The final assembly size is over 99% of the estimated genome size. We find good collinearity with the rice genome and have captured most of the gene space. Streptochaeta is similar to other grasses in the structure of its fruit (a caryopsis or grain) but has peculiar flowers and inflorescences that are distinct from those in the outgroups and in other grasses. To provide tools for investigations of floral structure, we analyzed two large families of transcription factors, AP2-like and R2R3 MYBs, that are known to control floral and spikelet development in rice and maize among other grasses. Many of these are also regulated by small RNAs. Structure of the gene trees showed that the well documented whole genome duplication at the origin of the grasses (ρ) occurred before the divergence of the Anomochlooideae lineage from the lineage leading to the rest of the grasses (the spikelet clade) and thus that the common ancestor of all grasses probably had two copies of the developmental genes. However, Streptochaeta (and by inference other members of Anomochlooideae) has lost one copy of many genes. The peculiar floral morphology of Streptochaeta may thus have derived from an ancestral plant that was morphologically similar to the spikelet-bearing grasses. We further identify 114 loci producing microRNAs and 89 loci generating phased, secondary siRNAs, classes of small RNAs known to be influential in transcriptional and post-transcriptional regulation of several plant functions.

Draft genome of the herbaceous bamboo Raddia distichophylla.

Bamboos are important nontimber forest plants widely distributed in the tropical and subtropical regions of Asia, Africa, America, and Pacific islands. They comprise the Bambusoideae in the grass family (Poaceae), including approximately 1700 described species in 127 genera. In spite of the widespread uses of bamboo for food, construction, and bioenergy, the gene repertoire of bamboo still remains largely unexplored. Raddia distichophylla (Schrad. ex Nees) Chase, belonging to the tribe Olyreae (Bambusoideae, Poaceae), is a diploid herbaceous bamboo with only slightly lignified stems. In this study, we report a draft genome assembly of the ∼589 Mb whole-genome sequence of R. distichophylla with a contig N50 length of 86.36 Kb. Repeat sequences account for ∼49.08% of the genome assembly, of which LTR retrotransposons occupy ∼35.99% of the whole genome. A total of 30,763 protein-coding genes were annotated in the R. distichophylla genome with an average transcript size of 2887 bp. Access to this herbaceous bamboo genome sequence will provide novel insights into biochemistry, molecular marker-assisted breeding programs, and germplasm conservation for bamboo species worldwide.

The sequence and analysis of duplication-rich human chromosome 16.

Human chromosome 16 features one of the highest levels of segmentally duplicated sequence among the human autosomes. We report here the 78,884,754 base pairs of finished chromosome 16 sequence, representing over 99.9% of its euchromatin. Manual annotation revealed 880 protein-coding genes confirmed by 1,670 aligned transcripts, 19 transfer RNA genes, 341 pseudogenes and three RNA pseudogenes. These genes include metallothionein, cadherin and iroquois gene families, as well as the disease genes for polycystic kidney disease and acute myelomonocytic leukaemia. Several large-scale structural polymorphisms spanning hundreds of kilobase pairs were identified and result in gene content differences among humans. Whereas the segmental duplications of chromosome 16 are enriched in the relatively gene-poor pericentromere of the p arm, some are involved in recent gene duplication and conversion events that are likely to have had an impact on the evolution of primates and human disease susceptibility.