Research in Mammalian Mastication

Ongoing research efforts in mammalian mastication have definedseveral broad areas of mutual interest to workers in the discipline.They are (1) interrelationships between masticatory movements,(2) actions of the masticatory muscles, (3) comparisons betweenmasticatory structures and functions, (4) developmental aspects,(5) comparisons between limbs and jaws, and (6) neurophysiologicconsiderations. The roles (potential and actual) of masticatorycentral pattern generators, cerebral "mastication areas," differentneural mechanisms between mammalian taxa, neurophysiologic/morphologicinteractions, and biochemical factors within the total milieuof mammalian mastication are discussed.     

The Ontogeny of Mammalian Mastication

Movements of the oral apparatus begin during the fetal periodand develop in a consistent order. Jaw openingappears first,followed by active jaw closure and tongue movements, lip movements,sucking, and finally masticatory movements. The later developingmovements appear prenatally in precocious mammals such as guineapigs and sheep, but are postnatal in altricial mammals suchas rats, hamsters and rabbits. The orderly development of oralbehavior is probably related to the progressive maturation ofthe nervous system and neuromuscular connections. Most newbornmammals feed exclusively bysuckling, a combination of the tongueworking against the nipple and negative pressure at the backof the oral cavity. Thetransition from suckling to masticationis gradual and involves considerable learning. In at least onespecies, the domestic pig, infant animals chew using a somewhatdifferent muscular contraction pattern from that of adults.Age changes in muscle action lines are the most likely explanationfor this difference. After being established in infancy, theprocess of mastication undergoes only minor changes in rateand relative muscle activity during the juvenile period. Throughoutontogeny there is a reciprocal relation between morphology andbehavior. While masticatory performance depends on structureat any given stage, it also has profound effects on furthermusculoskeletal growth and differentiation.     

Tooth Morphology in Fossil and Extant Lagomorpha (Mammalia) Reflects Different Mastication Patterns

The functional interpretation of the cheek teeth and the mastication cycle of Lagomorpha are deduced from various aspects of tooth morphology of fossil and extant species. Mastication is composed of an almost orthal shearing and transverse grinding in a lingual direction. Shearing blades are not only indicated by facets but as well by thickened enamel. A primary shearing blade (PSB) inherited from stem lagomorphs occurs in all examined species. It can be correlated with facets 1 and 5 (sensu Crompton 1971) and occurs in very few mammals; it is conspicuously absent in the sister-taxon Rodentia. A secondary shearing blade (SSB) occurs in derived Ochotonidae and two basal Leporidae (Romerolagus and Pronolagus) as a convergent pattern. In fossil ochotonids from Europe, the “lagicone structure” is gradually reduced in favor of the SSB. Thus, ochotonids strengthen the shearing ability, whereas most leporids favor the grinding function realized by the partial crenulation of the enamel band of the re-entrant folds. For the mastication cycle, the distinct phases were recognized, related to phase I of the tribosphenic model. The first movement (phase Ia) is directed almost orthally, the second (phase Ib) lingually. Only in Lepus europaeus was an additional phase detected, which might correspond to phase II.     

Preservation of Functional Mammalian Polysomes by Lyophilization

LYOPHILIZATION has been used to preserve viable microorganisms for extended periods of time¹. Recently, it has been reported that ribosomes functional in in vitro protein synthesis can be isolated from lyophilized fungi² and that lyophilized Escherichia coliribosomes retain fully their capacity for poly U-directed phenylalanine incorporation after 5 months storage at room temperature over P₂O₅ (ref. 3). We have now compared the sedimentation profiles and poly U-directed phenylalanine incorporating activity of three types of rabbit reticulocyte ribosome preparations immediately after isolation and after freezing or lyophilization and storage for various times.     

Morphology of the Mammalian Ossicula auditûs.

THE complete memoir on the small ear‐bones of the Mammals will hereafter be published in the Society's ‘Transactions’ with copious illustrations, whereby an excellent comparison of the various forms peculiar to and significant of groups may be instituted. Previously elsewhere ? I have given a short résumé respecting the material which has afforded the means of study of the series, with a brief reference to what has already been published on the internal auditory apparatus, and added a short notice concerning points among certain of the higher groups of the Mammalia. For the present abstract I shall therefore confine my remarks to the auditory ossicles of the following orders, viz.:–the Insectivora, the Chiroptera, the Cetaeea, the Sirenia, the Edentata, the Marsupialia, and the Monotremata.     

Functional Morphology of Stylophoran Echinoderms

The life orientation and mode of life of stylophorans are a subject of much ongoing debate. Examination of the ornamentation occurring both on the arm and theca in several cornutes and mitrates strongly supports the view that the life orientation was similar in all stylophorans and was ‘flat‐surface down’. The presence of an asymmetrical ornamentation adapted to hinder, or minimize, back slippage of the organism in all stylophorans gives strong support to their interpretation as mostly sessile organisms, feeding with the arm facing the current and the theca downstream. The examination of a wide array of thecal morphologies and sculpture patterns displayed by the various groups of cornutes and mitrates allows the identification of three main modes of life in stylophorans: (1) an epibenthic mode of life, with the theca as main anchor to the substrate (e.g. asymmetrical cornutes, Diamphidiocystis); (2) an epibenthic mode of life, with the arm as main anchor to the sediment (e.g. symmetrical cornutes, Peltocystis, primitive Mitrocystitida, some Kirkocystidae); (3) an infaunal mode of life, with the theca buried in a slightly inclined attitude (e.g. some Kirkocystidae, Mitrocystitida with cuesta‐shaped ribs). The partially buried mode of life of Lagynocystis is intermediate between 2 and 3.     

Functional Analyses of Mammalian Reovirus Nonstructural Protein uNS

Genome replication of reovirus occurs in cytoplasmic inclusion bodies called viral factories or viroplasms. The viral nonstructural protein uNS, encoded by genome segment M3, is not a component of mature virions, but is expressed to high levels in infected cells and is concentrated in the infected cell factory matrix. Recent studies have demonstrated that uNS plays a central role in forming the matrix of these structures, as well as in recruiting other components to them for putative roles in genome replication and particle assembly.     

Functional Divergence and Evolutionary Turnover in Mammalian Phosphoproteomes

Protein phosphorylation is a key mechanism to regulate protein functions. However, the contribution of this protein modification to species divergence is still largely unknown. Here, we studied the evolution of mammalian phosphoregulation by comparing the human and mouse phosphoproteomes. We found that 84% of the positions that are phosphorylated in one species or the other are conserved at the residue level. Twenty percent of these conserved sites are phosphorylated in both species. This proportion is 2.5 times more than expected by chance alone, suggesting that purifying selection is preserving phosphoregulation. However, we show that the majority of the sites that are conserved at the residue level are differentially phosphorylated between species. These sites likely result from false-negative identifications due to incomplete experimental coverage, false-positive identifications and non-functional sites. In addition, our results suggest that at least 5% of them are likely to be true differentially phosphorylated sites and may thus contribute to the divergence in phosphorylation networks between mouse and humans and this, despite residue conservation between orthologous proteins. We also showed that evolutionary turnover of phosphosites at adjacent positions (in a distance range of up to 40 amino acids) in human or mouse leads to an over estimation of the divergence in phosphoregulation between these two species. These sites tend to be phosphorylated by the same kinases, supporting the hypothesis that they are functionally redundant. Our results support the hypothesis that the evolutionary turnover of phosphorylation sites contributes to the divergence in phosphorylation profiles while preserving phosphoregulation. Overall, our study provides advanced analyses of mammalian phosphoproteomes and a framework for the study of their contribution to phenotypic evolution.