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The effects of live yeast (Saccharomyces cerevisiae) and ruminal environment on in vitro true digestibility of dry matter, organic matter, and fiber components were evaluated. Ruminally-cannulated beef steers (n = 4; BW = 520 ± 30 kg) were used in a cross-over design to serve as donors of ruminal content. In vitro substrates (6 grower and 6 finisher diets) and individual ingredients (steam-flaked corn, wet corn gluten feed, and old-world bluestem hay) were dehydrated (55o C), ground (1 mm), and incubated using a 2 × 2 factorial arrangement of treatments: a) the presence or not of yeast (8×109 CFU/animal-daily adjusted to a 70 L ruminal content volume), and b) two ruminal inoculums collected from donors fed a grower (50:50) or a finisher (92:08) diet (concentrate:forage). Four incubation batches (experimental unit) were performed, in which the initial two batches were performed followed by another two after animals switched diets and were adapted again during 21d. A Daisy in vitro incubation system was used, and samples incubated for 48h (39°C). The GLIMMIX procedure of SAS was used for the statistical analysis. No 3-way interactions (P ≥ 0.71) or 2-way interactions with substrate were observed (P ≥ 0.25). The in vitro true digestibility of DM, OM, NDF, and hemicellulose increased when yeast was added to the incubator containing a 50:50 ruminal content (Rumen × Yeast; P ≤ 0.02). The ADF digestibility increased (P = 0.02) with yeast addition regardless of ruminal content type (50:50 or 92:08). As expected, finisher substrate diets were more (P < 0.01) digestible than grower diets. The combination of live yeast and a specific ruminal environment seemed to be more important than substrate type, in which live yeast improved in vitro digestion within a 50:50 ruminal inoculum more evidently.

The effects of steam-flaked corn bulk density during grain adaptation phase on ruminal microbiome were evaluated. Crossbred-Angus ruminally cannulated steers (n = 6; BW = 405 ± 42 kg) were assigned to a randomized complete block design (block = body weight) to 1 of 2 grain adaptation strategies: 1) steam-flaked corn (SFC) bulk density of 335 g/L; and 2) 412 g/L. Steers were ad libitum fed, individually, during 6-7d phases, consisting of: HAY, followed by the STEP-UP1 through STEP-UP4, diets, respectively, in which roughage was gradually replaced with grain until FINISHER diet was fed. Respective SFC bulk densities were fed throughout STEP-UP diets, while the FINISHER diet consisted of 335 g/L strategy only for both groups. Ruminal fluid samples (100 mL) were collected on d-5 of each step, at 6h post-feeding for DNA extraction. Microbiome data were sequenced by Illumnia® NovaSeq™ 6000 (16S rRNA). The SFC bulk density did not affect (P > 0.50) the relative abundance (RA) for any taxonomy classification. Regardless of SFC bulk density, inclusion of grain throughout adaptation phases affected domain (P ≤ 0.03) when initial phases were compared to FINISHER. Phylum RA were affected (P ≤ 0.05) for Actinobacteria (27%), Bacteroidetes (11%), and Euryarchaeota (2%). Within Class RA were affected (P ≤ 0.04) for Clostridia (46%), Actinobacteria (27%), and Bacilli (5%). Order effects on RA were observed (P ≤ 0.04) for Clostridiales (45%), Coriobacteriales (25%), and Lactobacillales (4%). Within Family RA was affected (P ≤ 0.03) for Coriobacteriaceae (25%), Lachnospiraceae (27%), Ruminococcaceae (6%), and Lactobacillaceae (4%), while a tendency (P = 0.09) was observed for Veillonellaceae (1%). In Genus, RA was affected (P ≤ 0.01) for Olsenela (22%), Pediococcus (3%), and Butyrivibrio (3%). As steers advance through subsequent grain adaptation phases until the FINISHER, more meaningful ruminal microbiome changes are observed than SFC density change.

The effect of pre-treatment with fibrolytic enzymes [cellulase/xylanase (Trichoderma ressie)] of growing diets (high quality and low-quality) on ruminal microbiome relative abundance (RA) were evaluated. Ruminally cannulated beef steers (n = 5; BW = 520 ± 30 kg) were used in a 5×4 unbalanced Latin square design using a 2×2 factorial arrangement of treatments: (a) growing diet quality [high (HQ) and low (LQ)] and (b) enzyme inclusion (0 or 0.75 mL/kg of diet DM). Steers were individually fed ad libitum throughout four 21-d periods consisting of 14-d of adaptation and 7-d of collections. Ruminal fluid samples (100 mL) were collected on d-5 of each collection-period, at 6 h post feeding for DNA extraction and determination of microbial RA. Microbiome data were sequenced by Illumnia® NovaSeq™ 6000 (16S rRNA). Regardless of enzyme×diet quality interaction (P ≥ 0.11) or pre-treatment with enzyme (P ≥ 0.12), Domain RA was affected (P ≤ 0.04), in which LQ diets increased RA of Bacteria (93.25 vs. 86.80%) and decreased Archaea (6.75 vs. 13.20%). In Phylum, LQ diets decreased RA (P ≤ 0.04) of Euryarchaeota (6.75 vs. 13.21%), and increased Bacteroidetes (11.22 vs. 2.26%). Within Class, LQ diets decreased RA (P ≤ 0.04) of Clostridia (38.66 vs. 51.40%), Methanobacteria (6.75 vs. 13.21%), and increased Bacteroidia (10.62 and 1.47%). Within Order LQ diets, decreased RA (P ≤ 0.04) of Clostridiales (38.47 vs. 51.29%), Methanobacteriales (6.75 vs. 13.21%), and increased Bacteroidales (10.62 vs. 1.47%). In Family, LQ diets showed decreased RA (P ≤ 0.04) of Methanobacteriaceae (6.75 vs. 13.21%), Ruminococcaceae (6.71 vs. 2.18%), and increased Prevotellaceae (9.83 vs. 1.17%). In Genus, LQ diets showed increased RA (P ≤ 0.03) of Prevotella (9.61 vs. 1.10%). The dietary pre-treatment with fibrolytic enzymes seems to not dramatically affect RA of ruminal microbiome, while growing diet quality greatly influenced ruminal microbiome RA.

Genome-wide association studies have discovered hundreds of loci associated with complex brain disorders, but it remains unclear in which cell types these loci are active. Here we integrate genome-wide association study results with single-cell transcriptomic data from the entire mouse nervous system to systematically identify cell types underlying brain complex traits. We show that psychiatric disorders are predominantly associated with projecting excitatory and inhibitory neurons. Neurological diseases were associated with different cell types, which is consistent with other lines of evidence. Notably, Parkinson’s disease was genetically associated not only with cholinergic and monoaminergic neurons (which include dopaminergic neurons) but also with enteric neurons and oligodendrocytes. Using post-mortem brain transcriptomic data, we confirmed alterations in these cells, even at the earliest stages of disease progression. Our study provides an important framework for understanding the cellular basis of complex brain maladies, and reveals an unexpected role of oligodendrocytes in Parkinson’s disease. © 2020, The Author(s), under exclusive licence to Springer Nature America, Inc.

The use of portable X-ray fluorescence (PXRF) spectrometry to detect dietary external markers on processed or unprocessed bovine fecal specimens to estimate apparent total tract digestibility (ATTD) was evaluated. Seven beef crossbred, ruminally-cannulated steers (BW = 520 ± 30 kg) were used in a completely randomized design. Steers were individually fed a steam-flaked corn-based finishing diet ad libitum for 21-d. Degradable gel capsules containing 7.5 g of titanium dioxide and chromic oxide were placed inside the rumen through the ruminal-cannulae twice daily, throughout the experiment. Fecal samples were collected twice daily from d 14–21, stored frozen (-20oC), and thawed on d-21 to make a composite sample per steer/day. Wet chemistry analyses of fecal samples included atomic absorption for Cr and spectrophotometry for TiO2 and were compared with PXRF to determine the concentration of these markers on three types of fecal specimens: fresh (average 22.5 % DM), dried (60oC, 72 h), or dried/ground (1 mm). Data were analyzed using the GLIMMIX procedures of SAS with collection day as a repeated measure. Delta between ATTD estimated by wet chemistry and PXRF was not different from zero when using fresh fecal specimens analyzed for Cr (P = 0.40) or Ti (P = 0.14), while for dried or dried/ground specimens it differed (P ≤ 0.04), in which ATTD was underestimated by 3.6 and 1.1 percent units for Cr and Ti, respectively, when using PXRF. Concentration of Ti in fecal specimens was similar (P = 0.39) between analytical methodologies regardless of sample preparation. However, concentration of Cr was underestimated (P < 0.01) by 13% when PXRF was used in dried or dried/ground fecal samples compared to wet chemistry. Detection of Cr and Ti in fresh fecal samples with PXRF spectrometry seems to be a viable alternative to wet chemistry for estimating ATTD for finishing steers.

The scattering at a rectilinear step change in depth of a shallow-water vortex pair consisting of two patches of equal but opposite-signed vorticity is studied. Using the constants of motion, an explicit relationship is derived relating the angle of incidence to the refracted angle after crossing. A pair colliding with a step from deep water crosses the escarpment and subsequently propagates in shallow water refracted towards the normal to the escarpment. A pair colliding with a step from shallow water either crosses and propagates in deep water refracted away from the normal or, does not cross the step and is instead totally internally reflected by the escarpment. For large depth changes, numerical computations show that the coherence of the vortex pair is lost on encountering the escarpment.

The paper outlines a modelling technique that predicts how woven fabric constructions will change shape when subjected to the application of hydrostatic pressure. The initial specification required for the technique is in terms of a triangulated pattern for each fabric piece in the construction and an initial 3D location for each node. Fabric types are characterised by look-up tables describing the warp and weft tensile and shear behaviours. The modelling proceeds by seeking to find the minimum energy state of the construction where material strain energy is balanced by the energy required to inflate the construction by the application of hydrostatic energy. Examples are provided to illustrate the operation of the technique.

In this paper, a model for predicting the stresses within a strained anisotropic fabric is presented. The fabric is characterised by tensile test data in the warp and weft directions as well as shear behaviour. The data is in the form of look-up tables that are capable of representing the non-linear form of each response. The model operates simultaneously on a triangular representation for both the 2D pattern defined on an undistorted fabric and an equivalent triangulation of a 3D representation of the fabric. An energy relaxation technique is applied to the situation in order to predict how the 3D representation should react in order to reach a minimum energy state. Arbitrary constraining configurations can be applied by setting a flag for each node to indicate those fixed in 3D. The resultant forces on fixed nodes can be determined. The model has been successfully applied to two examples. In the second of these, validation took the form of measuring the physical force effects of the fabric on an experimental rig and comparing them with those predicted by the model. It was then demonstrated that the model could predict other forces that could not be easily measured.

Machining with an uneven chip load can lead to dimensional errors on workpiece surfaces. Tool deflections are liable to occur especially when machining with slender end mills where tool flexibility is high compared with that of the machine structure. Whilst steady state machining errors are commonly considered, it is more difficult to deal with the transient errors that occur at tool entry and exit to cuts. Using a predictive equation that links tool deflection to chip load, it is possible to compare expected errors of form or position with standards set out using geometric dimensioning and tolerancing. Machining conditions can be investigated to establish whether part features can be manufactured within tolerances allowed. A process for selecting machining conditions for producing a circular pocket is introduced. The use of a tolerance analysis chart is demonstrated to clarify the results of machining at tool entry and exit.

While considerable progress has been made in fabric modelling techniques, which could be used in garment computer aided design (CAD) systems, less attention has been paid to the way in which garments might be specified in a CAD system. The final shape taken by a garment is often achieved through the incorporation of darts, seams, edges, stiffening pads and local stretch of the fabric. In order to gain credibility, CAD systems should have to functionally handle the level of complexity normally found in garment assemblies combined with a simple interface to specify the constructional detail. Different conceptual approaches are possible. Given an accurate drape algorithm, garment block patterns in a chosen fabric could simply be anchored or attached around the mannequin in order to achieve a visualisation of a garment. If changes were necessary, the user would alter the 2D patterns and re-run the visualisation. An alternative and possibly more productive approach would be to specify in 3D, with advanced drawing tools, the 3D shape required. Processing of the 3D garment piece using expert rules would indicate the 2D shape and constructional detail required to achieve the final form. This paper describes a CAD system that is under development and which aims to facilitate both the approaches.

Applications such as garment manufacture and composite structure fabrication require a two dimensional (2D) woven material to assume a three dimensional (3D) shape. The specification of the process is usually initiated by defining the 3D surface. Hence, the problem arises of determining the best 2D pattern. The problem is made more complex by the anistropic nature of woven fabrics which are often used as raw material. Such materials display a variation in mechanical properties with respect to the woven structure. This paper presents a model for determining the optimum 2D pattern for a specified 3D surface where optimality is determined in terms of minimising the energy distribution required to force the 2D pattern to assume the 3D shape. The 3D surface specification is assumed to consist of a polygonal mesh. The model allows affine transformations to be applied to the weave structure which can be unique for each polygon in the mesh. Important considerations in the modelling process include the following: 1. The degree to which the specified 3D surface departs from a developable surface. 2. The energy components used to model the woven structure and their sensitivity to weave direction. Essentially, these stem from tensile strain in each direction of the weave and shear strain. 3. The prediction of weave geometry as it reacts to the energy distribution being applied. The model is demonstrated by applying it to a relatively simple pyramidal 3D shape. Energy values are optimised to produce a pattern that requires the minimum overall energy to be applied to the 2D pattern in order for it to assume the 3D shape. This 2D pattern is sensitive to the orientation of the woven structure with predictions being made of how the woven structure will behave in 3D.

A component of the direct operating cost of aircraft is that associated with the manufacturing cost. This affects depreciation, interest, insurance and maintenance charges. By relaxing the requirements for aerodynamic surface smoothness the manufacturing cost can be reduced at the expense of an increase in drag and corresponding fuel costs. This work is part of a study to examine this multidisciplinary problem. Only isolated turbofan nacelles are considered. The costs associated with assigning different tolerance levels to the feature dimensions on nacelles are assessed. A statistical procedure is employed to estimate the cost-tolerance relationship for eleven features involving gaps, steps, surface profile and fastener flushness. This procedure requires actual manufacturing and cost source data. A knowledge of the cost-tolerance relationships is useful in a concurrent engineering context. It will allow aerodynamicists to optimise surface smoothness in consultation with production engineers, thus achieving the best compromise between cost and drag.

[1] Laboratory experiments are performed which study the behavior of two-dimensional vortex dipoles propagating toward a rectilinear step change in depth. The dipoles are formed in a homogeneous shallow water layer by an impulsive jet and approach the step from either relatively deep or shallow water at normal or oblique incidence. Qualitative observations agree well with previous theoretical predictions: At normal incidence the separation of the two vortices forming the dipole increases for a dipole crossing from deep water and decreases for a dipole crossing from shallow water. Dipoles with oblique angles of incidence refract toward the normal when passing into shallower water, and dipoles that cross the step into deep water refract away from the normal. Dipoles approaching from shallow water at sufficiently large angles of incidence reflect off the step.

Knowledge of the complete genomic DNA sequence of an organism allows a systematic approach to defining its genetic components. The genomic sequence provides access to the complete structures of all genes, including those without known function, their control elements, and, by inference, the proteins they encode, as well as all other biologically important sequences. Furthermore, the sequence is a rich and permanent source of information for the design of further biological studies of the organism and for the study of evolution through cross-species sequence comparison. The power of this approach has been amply demonstrated by the determination of the sequences of a number of microbial and model organisms. The next step is to obtain the complete sequence of the entire human genome. Here we report the sequence of the euchromatic part of human chromosome 22. The sequence obtained consists of 12 contiguous segments spanning 33.4 megabases, contains at least 545 genes and 134 pseudogenes, and provides the first view of the complex chromosomal landscapes that will be found in the rest of the genome.

The immunoglobulin (Ig) heavy chain variable (VH) gene family of Heterodontus francisci (horned shark), a phylogenetically distant vertebrate, is unique in that VH, diversity (DH), joining (JH) and constant region (CH) gene segments are linked closely, in multiple individual clusters. The V regions of 12 genomic (liver and gonad) DNA clones have been sequenced completely and three organization patterns are evident: (i) VH-D1-D2-JH-CH with unique 12/22 and 12/12 spacers in the respective D recombination signal sequences (RSSs); VH and JH segments have 23 nucleotide (nt) spacers, (ii) VHDH-JH-CH, an unusual germline configuration with joined VH and DH segments and (iii) VHDHJH-CH, with all segmental elements being joined. The latter two configurations do not appear to be pseudogenes. Another VH-D1-D2-JH-CH gene possesses a D1 segment that is flanked by RSSs with 12 nt spacers and a D2 segment with 22/12 spacers. Based on the comparison of spleen, VH+ cDNA sequences to a germline consensus, it is evident that both DH segments as well as junctional and N-type diversity account for Ig variability. In this early vertebrate, the Ig genes share unique properties with higher vertebrate T-cell receptor as well as with Ig and may reflect the structure of a common ancestral antigen binding receptor gene.

An enzyme-linked immunosorbent assay for Treponema pallidum antibody (Syphilis Bio-EnzaBead Kit; Organon Teknika Corp., formerly Litton Bionetics, Kensington, Md.) was compared with the standard fluorescent treponemal antibody absorption test for syphilis (indirect fluorescent-antibody confirmatory test; Zeus Scientific Inc., Raritan, N.J.). Six hundred specimens tested in the rapid plasma Reagin card test (Hynson, Westcott and Dunning, Inc., Baltimore, Md.) and microhemagglutination assay for antibodies to T. pallidum (Ames Division, Miles Laboratories, Inc., Elkhart, Ind.) were used for comparison testing. One hundred and sixty-two specimens were from either persons with syphilis or persons whose history was unknown but who showed reactive serology, whereas 438 were from persons without syphilis. The reactivity of each serum by the Bio-EnzaBead test was determined visually and from optical density readings. Excellent reproducibility of visual readings was obtained; all results were within +/- 1 gradation of the expected readings in all 60 coded sera tested. Overall agreement between the Bio-EnzaBead and fluorescent treponemal antibody absorption test results was 96.3% when read visually and greater than or equal to 95.7% when using optical density readings. Our data indicate that Bio-EnzaBead results read at 405 nm and determined by using the mean antigen optical density reading of the nonreactive control plus 0.025 as the cutoff value provide an overall sensitivity and specificity of 93.0 and 98.6%, respectively, along with the best agreement with the fluorescent treponemal antibody absorption assay. Ease of performance and objectivity also contribute toward the acceptability of this assay as an alternative confirmatory test for syphilis.

In mammals, the immunoglobulin heavy-chain variable region (VH) locus is organized in a linear fashion; individual VH, diversity (DH), joining (JH) and constant (CH) region segments are linked in separate regions. During somatic development, coding segments flanked by characteristic short recombination signal sequences, separated by intervening sequence regions that may exceed 2,000 kilobases (kb), are recombined. Combinatorial joining of different segments as well as imprecision in this process contribute to the diversity of the primary antibody response; subsequent mutation further alters functionally rearranged genes. This basic somatic reorganization mechanism is shared by six major families of genes encoding antigen receptors. Previously, we have shown that multiple germline genes and mammalian-like recombination signal sequences are associated with the VH gene family of Heterodontus francisci (horned shark), a primitive elasmobranch. Studies presented here demonstrate that segmental reorganization involving mammalian-like DH and JH segments occurs in the lymphoid tissues of this species. In marked contrast to the mammalian system, we find multiple instances of close linkage (approximately 10 kb) between individual VH, DH, JH, and CH segments. This unique organization may limit combinatorial joining and be a factor in the restricted antibody response of this lower vertebrate.

Complete nucleotide sequences are described for three caiman (Caiman crocodylus crocodylus) immunoglobulin VH genes (C3, E1, and G4) that hybridize with a murine VH probe. The E1 and G4 genes are physically linked (intergenic distance, approximately equal to 6.5 kilobases) in the same transcriptional orientation but are not directly contiguous with the C3 gene. When the coding segments, including both framework and complementarity-determining regions, of these genes and the murine probe sequences are compared by metric analysis, it is apparent that the caiman genes are only slightly more related to each other than to the mammalian sequence, consistent with significant preservation of nucleotide sequence over an extended period of phylogenetic time. Based on the presence of transcriptionally critical 5' sequences and the absence of terminator codons, frameshift mutations, or other recognizable alterations, the genes do not appear to be pseudogenes. The E1 gene, however, is distinguished from other VH genes because (i) the spacer region within the 3' recombination signal sequence is 12 base pairs, typical of VK genes but not of VH genes, which possess 22- to 23-base-pair spacers and (ii) a near-perfect VH recombination signal sequence is present within the intervening sequence that splits the segment encoding the leader. These studies establish VH gene multiplicity in a species that arose prior to mammalian radiation and provide a description of differences in the configuration and location of recombination elements associated with an otherwise potentially functional gene.

Immunoglobulin variable (V) gene regions1–10 typify extensive multigenic families in terms of overall size, chromosomal arrangement and presence of large numbers of apparent pseudogenes. A unique mechanism of somatic reorganization involving recombination of VH, D and JH or VL and JL segments accompanies the differentiation of lymphoid cells5,6,11,12 and together with somatic mutation6,13,14 and other types of recombination15,16 accounts for V-region diversity. Although these processes have been well characterized in higher mammals, little is known concerning their origin and diversification during phylogenetic time. Previously, we described the blot-hybridization characteristics of murine VHIII probes with restriction enzyme-digested genomic DNA isolated from several phylogenetically critical species, including Caiman crocodylus, a modern representative of an ancient reptilian subclass17. Here we have used a murine probe, S107V, to select homologous clones from a library of Caiman genomic DNA constructed in a λ bacteriophage. The complete nucleotide sequence of a Caiman gene homologous to the murine VH gene and its adjacent 5′ and 3′ region is described. Comparison of the sequence with mammalian prototypes shows evidence of considerable organizational and structural homology extending outside the presumed VH-coding region and including elements believed to be involved in somatic recombination. Inferences about the evolution of this multigenic family can now be extended to the level of phylogenetic class.

A homologous probe for the constant region of the Heterodontus francisci (horned shark) immunoglobulin heavy chain was used to screen a genomic DNA library constructed in bacteriophage lambda, and a large number of independent clones were recovered. Their hybridization patterns with segment-specific probes are consistent with the close linkage of heavy-chain constant (CH), joining (JH), and variable (VH) gene segments. Differences in the nucleotide sequences of the first CH exon of five genes primarily are localized to 5' positions; extended regions of sequence identity are noted at 3' positions. The predicted amino acid sequences of each gene are different and are related distantly to the corresponding regions of higher vertebrate immunoglobulins. Gene-specific oligodeoxynucleotide probes were used to establish that at least three of the five genes are transcriptionally active. Quantitative gene titration data are consistent with the large numbers of genes suggested by the library screening analyses. In this representative early vertebrate, it appears that (VH-diversity-JH) segments are associated with individual constant region genes that can differ at the predicted protein level.

A mouse VH probe has been used to identify and isolate VH homologs in a DNA library of Heterodontus francisci (horned shark). The complete nucleotide sequence of one VH gene, HXIA, has been determined and found to exhibit striking organizational homology and nucleotide identity with mammalian prototype VH genes. Metric analysis of the complete sequence is consistent with the early phylogenetic diversification of framework and complementarity-determining regions (CDR). Both the predicted amino acid sequence and the specific hybridization of the CDR2-specific, synthetic oligodeoxynucleotide probe in spleen mRNA suggest that HXIA is functionally expressed. A probe consisting of the entire coding region of this gene hybridizes with multiple components in Southern blot analysis of Heterodontus genomic DNA and together with the identification of additional unique VH+-lambda clones indicates that considerable complexity is associated with the germline VH gene family in a contemporary species that represents an early stage in the phylogenetic development of the vertebrates.

The Dean, Hibbsville, and Decatur bridges were constructed in southern Iowa during 1994. Each bridge is a simple span, stress laminated deck superstructure, approximately 7.3 m (24 ft) long, constructed from eastern cottonwood lumber. The performance of each bridge was monitored for approximately 2 years, beginning shortly after installation. Monitoring involved collecting and evaluating data pertaining to the moisture content and vertical creep of the wood decks, the force level of the stressing bars, and the behavior of the bridges under static load conditions. Comprehensive visual inspections were also conducted to assess the overall condition of the structure. Based on field evaluations, the bridges are performing well with minor serviceability deficiencies.

Immunoglobulin (Ig) gene organization in Heterodontus francisci (horned shark), a phylogenetically primitive vertebrate, is unique. Homologous Ig heavy chain variable (VH) and constant region (CH) specific probes were used to screen a spleen cDNA library constructed in lambda gt11. Both secretory (SEC) and transmembrane (TM) cDNA clones were recovered; the latter were identified by a negative selection strategy. The complete sequence of the CH portion of a Heterodontus genomic DNA-lambda clone also was determined. The sequences of the individual CH genes differ from each other in all exons. When compared to mammalian prototypes, similarities in exon and intron organization as well as conservation of sequences involved with differential splicing of SEC and TM mRNA indicate that Heterodontus heavy chain genes are of the mu type, although intron lengths are uniformly longer in Heterodontus. Heterodontus genes are not associated, however, with the family of DNA sequences that have been implicated in heavy chain class switching in mammals. Spleen cDNA library screening and RNA blot analyses indicate that mRNAs encoding TM Ig are exceedingly rare. The relationship between this quantitative difference and the distribution of polyadenylation signal sequences suggests that regulation of Ig gene expression in Heterodontus may be highly dependent on position effects.