CARDIAC MUSCLE OF THE HORSESHOE CRAB, LIMULUS POLYPHEMUS : I. Ultrastructure

The fine structure of the cardiac muscle of the horseshoe crab, Limulus polyphemus, has been studied with respect to the organization of its contractile material, and the structure of its organelles and the cell junctions. Longitudinal sections show long sarcomeres (5.37 µ at L_max), wide A bands (2.7 µ), irregular Z lines, no M line, and no apparent H zone. Transverse sections through the S zone of the A band show that each thick filament is ca. 180 A in diameter, is circular in profile with a center of low density, and is surrounded by an orbit of 9–12 thin filaments, each 60 A in diameter. Thick filaments are confined to the A band: thin filaments originate at the Z band, extend through the I band, and pass into the A band between the thick filaments. The sarcolemmal surface area is increased significantly by intercellular clefts. Extending into the fiber from these clefts and from the sarcolemma, T tubules pass into the fiber at the A-I level. Each fibril is enveloped by a profuse membranous covering of sarcoplasmic reticulum (SR). Sacculations of the SR occur at the A-I boundary where they make diadic contact with longitudinal branches of the T system. These branches also extend toward the Z, enlarge at the Z line, and pass into the next sarcomere. Infrequently noted were intercalated discs possessing terminal insertion and desmosome modifications, but lacking close junctions (fasciae occludentes). These structural details are compared with those of mammalian cardiac and invertebrate muscles.     

A SPECIATIONAL HISTORY OF “LIVING FOSSILS”: MOLECULAR EVOLUTIONARY PATTERNS IN HORSESHOE CRABS

Horseshoe crabs' exceptional morphological conservatism over the past 150 My has led to their reputation as “living fossils,” but also has served to obscure phylogenetic relationships within the complex. Here we employ nucleotide sequences from two mitochondrial genes to assess molecular evolutionary rates and patterns among all extant horseshoe crab species. The American species Limulus polyphemus proved to be the sister taxon to a clade composed of the Asiatic species Tachypleus gigas, T. tridentatus, and Carcinoscorpius rotundicauda, whose relationships inter se were not resolved definitively. Both absolute and relative rate tests suggest a moderate slowdown in sequence evolution in horseshoe crabs. Nonetheless, dates of the lineage separations remain uncertain primarily because of reservations about molecular-clock calibrations resulting from large rate variances at examined loci across Arthropods and other animal lineages, as inferred in this and prior studies. Thus, ironically, separation dates as estimated by molecular evidence in general may remain most insecure in taxonomic groups for which such information is needed most—those lacking strong biogeographic or fossil benchmarks for internal-clock calibrations. In any event, the current results show that large numbers of molecular characters distinguish even these most morphologically conservative of organisms. Furthermore, comparisons against previously published mitochondrial sequence data in the morphologically dynamic hermit crab–king crab complex demonstrates that striking heterogeneity in levels of morphotypic differentiation can characterize Arthropod lineages at similar magnitudes of molecular divergence.     

PHYLOGENETIC AFFINITIES OF “CHANTRANSIA” STAGES IN MEMBERS OF THE BATRACHOSPERMALES AND THOREALES (RHODOPHYTA)1

This study evaluated the phylogenetic relationship among samples of “Chantransia” stage of the Batrachospermales and Thoreales from several regions of the world based on sequences of two genes—the plastid‐encoded RUBISCO LSU gene (rbcL) and the nuclear SSU ribosomal DNA gene (SSU rDNA). All sequences of “Chantransia macrospora” were shown to belong to Batrachospermum macrosporum based on both molecular markers, confirming evidence from previous studies. In contrast, nine species are now associated with “Chantransia pygmaea,” including seven species of the Batrachospermales and two of the Thoreales. Therefore, the presence of “C. macrospora” in a stream can be considered reliable evidence that it belongs to B. macrosporum, whereas the occurrence of “C. pygmaea” does not allow the recognition of any particular species, since it is associated with at least nine species. Affinities of “Chantransia” stages to particular taxa were congruent for 70.5% of the samples comparing the rbcL and SSU analyses, which were associated with the same or closely related species for both markers. Sequence divergences have been reported in the “Chantransia” stage in comparison to the respective gametophyte, and this matter deserves further attention.     

GEOGRAPHIC VARIATION IN ALLOZYMES IN A “RING SPECIES,” THE PLETHODONTID SALAMANDER ENSATINA ESCHSCHOLTZII OF WESTERN NORTH AMERICA

The ring species Ensatina eschscholtzii (a plethodontid salamander) of western North America has a circle of subspecies surrounding the Central Valley of California which come into contact and are sympatric in southern California. We examined 26 proteins in 19 populations (maximum of 10 specimens per population) collected throughout the range in order to gain an understanding of the degree of differentiation in the group. Allozymic differentiation is profound, with genetic distances in excess of 0.5 (Rogers or Nei) between populations. Naturally hybridizing populations differ by genetic distances greater than 0.4. Two general classes of color morphs, blotched and unblotched, are segregated geographically, but they do not form discrete genetic units. Both are deeply differentiated, and genetic distances among populations of either class exceed those measured between the classes where they are sympatric in southern California. This study disclosed little evidence of gene exchange around the ring of populations and sampling of many additional populations in regions between populations sampled thus far will be required to determine whether smooth intergradation occurs. Although genetic distances measured exceed those between some co-occurring species of plethodontid salamanders, we find no evidence of borders between cryptic species.     

FLAT REACTION NORMS AND “FROZEN” PHENOTYPIC VARIATION IN CLONAL SNAILS (POTAMOPYRGUS ANTIPODARUM)

The Frozen Niche-Variation hypothesis (FNV) suggests that clones randomly sample and “freeze” the genotypes of their ancestral sexual populations. Hence, each clone expresses only a fraction of the total niche-use variation observed in the sexual population, which may lead to selection for ecological specialization and coexistence of clones. A generalized form of the FNV model suggests that the same is true for life-history (as well as other) traits that have important fitness consequences, but do not relate directly to niche use. We refer to the general form of the model as the Frozen Phenotypic Variation (FPV) model. A mixed population of sexual and parthenogenetic snails (Potamopyrgus antipodarum) in a New Zealand lake allowed us to examine the phenotypic variation expressed by coexisting clones in two benthic habitats, and to compare that variation to the sexual population. Three clones were found primarily in an aquatic macrophyte zone composed of Isoetes kirkii (1.5–3.0 m deep), and three additional clones were found in a deeper macrophyte zone composed of Elodea canadensis (4.0–6.0 m deep). These clones showed significant variation between habitats, which mirrored that observed in the sexual population. Specifically, clones and sexuals from the deeper habitat matured at a larger size and had larger broods. There was also significant among-clone variation within habitats; and as expected under the FPV model, the within-clone coefficients of variation for size at maturity were low in both habitats when compared to the sexual population. In addition, we found four clones that were common in both macrophyte zones. The reaction norms of these clones were flat across habitats, suggesting little phenotypic plasticity for morphology or life-history traits. Flat reaction norms, high among-clone variation, and low coefficients of variation (relative to the sexual population) are in accordance with the FPV model for the origin of clonal lineages. We also measured the prevalence of infection by trematode larvae to determine whether clones are inherently more or less infectable, or whether they are freezing phenotypic variation for resistance from the sexual population. We did this in the deep habitats of the lake where recycling of the parasite by the vertebrate host is unlikely, thereby reducing the complications raised by frequency-dependent responses of parasites to host genotypes. We found no indication that clones are either more or less infectable than the resident sexual population. Taken together, our results suggest that phenotypic variation for both life-history traits and resistance to parasites is frozen by clones from the local sexual population.     

“Laughter” and “Smile” in Barbary Macaques (Macaca sylvanus)

In an observational study on semi-free Barbary macaques it was investigated whether the phylogenetic roots of human laughter and smile can be traced back to the genus Macaca. On the basis of morphological similarity a ‘relaxed open-mouth display’ as the phylogenetic precursor of the laughter, and a ‘silent bared-teeth display’ as the possible ancestor of the smile can be distinguished in the repertory of the Barbary macaque. Behavioural sequences from focal animal protocols were analyzed in order to establish message and meaning of both displays. Relaxed open-mouth display is regularly observed in the play interactions of juveniles. It is associated with partner-directed behaviour, it is frequently answered by a relaxed open-mouth display of the receiver, and accompanied by a special vocalization. Although up to 50% of the juvenile's play partners were higher ranking than themselves voluntary participation was the rule. Most characteristically, the behaviour patterns shown by both play partners are highly symmetrical and synchronized. Silent bared-teeth display is typically accompanied by evasive or submissive body movements, and occurs primarily in dyadic interactions, mainly by the lower ranking individual. It is not an unidirectional sign of a linear dominance hierarchy, though. Silent bared-teeth display is a frequent answer to aggressive behaviour shown by the receiver. After its performance, an increase of body contact between sender and receiver was observed. Behavioural sequences of senders and receivers are complementary, but lose their asymmetry after occurrence of the display. It is concluded that these results further support Van Hooff 's (1972) view that human laughter and smile have different phylogenetic roots: while silent bared-teeth display is a signal of submission and appeasement, relaxed open-mouth display is rightly called the ‘play face’, and is an expression of fun.     

PHOTOADAPTATION AND THE “PACKAGE” EFFECT IN DUNALIELLA TERTIOLECTA (CHLOROPHYCEAE)1

In the marine unicellular chlorophyte, Dunaliella tertiolecta Butcher, the spectrally averaged m vivo absorption cross section, normalized to chlorophyll a (so-called a* values), vary two-fold in response to changes in growth irradiance. We used a kinetic approach to examine the specific factors which account for these changes in optical properties as cells photoadapt. Using Triton X-100 to solubilize membranes, we were able to differentiate between “package” effects and pigmentation effects. Our analyses suggest that 43–49% of the variability in a* is due to changes in pigmentation, whereas 51–57% is due to the “package” effect. Further analyses revealed that changes in cell sue did not significantly affect packaging, while thylakoid stacking and the transparency of thylakoid membranes were important factors. Our results suggest that thylakoid membrane protein/lipid ratios change during photoadaptation, and these changes influence the effective rate of light harvesting per unit chlorophyll a.