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Negative frequency dependent selection maintains shell banding polymorphisms in two marine snails (Littorina fabalis and L. saxatilis)
<p>The presence of shell bands is common in gastropods. The marine snails, <i>Littorina fabalis</i> and <i>L. saxatilis</i>, are<i> </i>both polymorphic for this trait. Such polymorphism would be expected to be lost by the action of genetic drift or directional selection, but it appears to be widespread at relatively constant frequencies. This suggests it is maintained by balancing selection on the trait or on a genetically linked trait. Using long time-series of empirical data, we compared potential effects of genetic drift and negative frequency-dependent selection, in the two species. The contribution of genetic drift to changes in the frequency of bands in <i>L. fabalis</i> was estimated using the effective population size estimated from microsatellite data, while the effect of genetic drift in <i>L. saxatilis</i> were derived from previously published study. Frequency-dependent selection was assessed comparing the cross-product estimator of fitness with the frequency of the polymorphism across years using a regression analysis. Both studied species showed patterns of negative frequency-dependent selection. In addition, in <i>L. fabalis</i>, contributions from genetic drift could explain some of the changes in banding frequency. Overdominance and heterogeneous selection did not fit well to our data. The possible biological explanations resulting on the maintenance of the banding polymorphism are discussed.</p>
Figure 3 in Combined threats to native smooth-shelled mussels (genus Mytilus) in Australia: bioinvasions and hybridization
Figure 3. Discriminant analysis of principal component (DAPC) plot of SNP variation amongst Australian mussels with the inclusion of reference Northern and Southern hemisphere populations of Mytilus spp. sampling groups: (1) M. galloprovincialis, (2) Mytilus spp.: samples from New Zealand mainland with some Australian individuals, mainly from Port Arthur, Tasmania, (3) M. platensis, (4) M. chilensis, (5) M. edulis, (6) M. galloprovincialis × M. planulatus hybrids from Australia and (7) New Zealand offshore island samples (AUCB and CAMI). For details of group membership refer to Supporting Information, Table S6.
Figure 2 in Combined threats to native smooth-shelled mussels (genus Mytilus) in Australia: bioinvasions and hybridization
Figure 2. Neighbour joining (NJ) tree of the ten mussel samples from Australia, three from New Zealand and reference samples of Mytilus galloprovincialis, M. edulis, M. platensis, M. chilensis and M. trossulus, based on the FST matrix from allele frequencies of the SNP loci. NJ tree obtained with POPTREEW and visualized with MEGA v.6. Population/sample codes as shown in Table 1. For the Australian samples: -W, wild; -F, farmed. FST matrix details are presented in Supporting Information, Table S5.
Figure 2 in The potential for using shell proteins in gastropod systematics, assessed in patellogastropod limpets
Figure 2. PCA analysis of the ratio of the numbers of peptide spectrum matches in individual proteins to the number in B3A0P1. The x and y axes show principal component 2 and principal component 3 that respectively explain 11% and 8.1% of the total variance. PCA1 (not graphed) explains 46.81% of the variation. Prediction curves are drawn so that with probability 0.95, a new observation from the same group will fall inside the ellipse. The legend identifications are: Lm: Lottia cf. mixta; Np: Notoacmea petterdi; Nsp: Notoacmea sp.; Pl: Patelloida latistrigata; Pm: Patelloida mimula; Sc: Scutellastra chapmani; Sp: Scutellastra peronii; Ct: Cellana tramoserica; and Sd: Siphonaria diemenensis.
Figure 3 in The potential for using shell proteins in gastropod systematics, assessed in patellogastropod limpets
Figure 3. Peptide profiles from abundant Lottia gigantea proteins. Red-coloured cells signify peptide absence, green presence, light green that the peptide is absent from one specimen of the species and yellow that it is absent from two specimens. Taxa are identified as follows: Sd: Siphonaria diemenensis; Ct: Cellana tramoserica; Lm: Lottia cf. mixta; Np; Notoacmea petterdi; Ns: Notoacmea sp.; Pl: Patelloida latistrigata; Pm: Patelloida mimula; Sc: Scutellastra chapmani; and Sp: Scutellastra peronii.
Figure 1 in The potential for using shell proteins in gastropod systematics, assessed in patellogastropod limpets
Figure 1. The phylogenetic relationships of gastropod peroxidases. Sequences from Lottia gigantea ("Lgig" prefix) and Patelloida mimula ("Pmim" prefix) are in bold font. Sequences from other species are identified by the following prefixes: Aplysia californica, Acal; Biomphalaria glabrata (Say, 1818), Bgla; Cepaea nemoralis, Cnem; Elysia chlorotica Gould, 1870, Echlo; Haliotis discus Reeve, 1846, Hdis; Lymnaea stagnalis, Lsta; Pomacea canaliculata, Pcan; Reishia bronni (Dunker, 1860), Rbro. Bootstrap values over 70 are shown above branches. Proteins known to be associated with the shell proteome, or suggested here to be possibly so, are indicated by shaded boxes, the lighter shade indicating a possible association. Branches shown with thicker lines are seen in both the overall and Gblocks filtered aligments and those with thinner lines only in the overall alignment. Clades composed of members of a single principal gastropod group are identified as follows: Patelloida, P; Vetigastropoda, V; Caenogastropoda, C; and Heterobranchia, H. For spacing reasons, two such designations are indicated by vertical bars to the right of sequence identifications. The large clade, marked by an asterisk, with low support, and the two small clades with members from multiple gastropod groups, marked by a cross, are discussed in the text.
Figure 7 in Combined threats to native smooth-shelled mussels (genus Mytilus) in Australia: bioinvasions and hybridization
Figure 7. Structure plot (K = 3) for Australian 'pure' native Mytilus planulatus pooled as one group (Austr.), two reference Northern hemisphere M. galloprovincialis samples (CAM, ORI), mainland New Zealand (AKAR) and offshore island lineages (AUCB, CAMI) of M. aoteanus. Plots constructed based on Australian individuals without admixture, identified by STRUCTURE and assigned to the M. planulatus cluster (q> 0.8). Each individual is represented by a single vertical line, samples are separated by a black vertical line, and site abbreviations (Table 1) are given along with reference taxon names.
Figure 4 in Combined threats to native smooth-shelled mussels (genus Mytilus) in Australia: bioinvasions and hybridization
Figure 4. Discriminant analysis of principal component (DAPC) plot of SNP variation amongst Australian mussels with the inclusion of reference Northern and Southern hemisphere populations of Mytilus spp. Sampling groups: (1) New Zealand mainland with some Australian individuals, mainly from Port Arthur, Tasmania, (2) reference M. galloprovincialis from the Mediterranean Sea and the North Atlantic Ocean with some Australian individuals, (3) New Zealand offshore island samples, (4) mixed status mussels from Australia (individuals from all ten sampled Australian sites) and some reference M. galloprovincialis from the Mediterranean Sea. For details of group membership refer to Supporting Information, Table S7.
Figure 1 in Combined threats to native smooth-shelled mussels (genus Mytilus) in Australia: bioinvasions and hybridization
Figure 1. Geographic locations of ten mussel sampling sites in Australia. See Table 1 for details of site codes, numbers of individuals per location, and origin: -W, wild; -F, farmed mussels.
Ptychographic X-ray spackle tracking (PXST) scan of the biomineralized shell of a marine planktonic diatom
<p>The PXST scan of the biomineralized shell of a marine planktonic diatom sample measured at P11 beamtime of the PETRA III synchrotron radiation facility. The beam was focused with a pair of MLLs with focal lengths of 1.25 mm and 1.15 mm and numerical apertures of 0.014 and 0.015, in the vertical and horizontal directions, respectively. The X-ray beam photon energy was 17.5 keV.</p>
Data from: Correlation of shell phenotype and local environment suggests a role for natural selection in the evolution of Placostylus snails
The giant edible Placostylus snails of New Caledonia occur across a wide range of environmental conditions, from the dry southwest to the wetter central and northeastern regions. In large, slow-moving animals such as Placostylus, speciation could be assumed to be largely driven by allopatry and genetic drift as opposed to natural selection. We examined variation in shell morphology using geometric morphometrics and genetic structure within two species of Placostylus (P. fibratus, P. porphyrostomus), to determine the drivers of diversity in this group. Despite the current patchy distribution of snails on New Caledonia, both mtDNA and nuclear SNP data sets (>3000 loci) showed weak admixing between populations and species. Shell morphology was concordant with the genetic clusters we identified and had a strong relationship with local environment. The genetic data, in contrast to the morphological data, did not show concordance with climatic conditions, suggesting the snails are not limited in their ability to adapt to different environments. In sympatry, P. fibratus and P. porphyrostomus maintained genetic and morphological differences, suggesting a genetic basis of phenotypic variation. Convergence of shell shape was observed in two adjacent populations that are genetically isolated but experience similar habitat and climatic conditions. Conversely, some populations in contrasting environments were morphologically distinct although genetically indistinguishable. We infer that morphological divergence in the Placostylus snails of New Caledonia is mediated by adaptation to the local environment.
FIGURE 14 in Three new species of shell-boring Dipolydora (Annelida: Spionidae: Polydorini) from the South China Sea and the Gulf of Thailand, Vietnam, with comments on the modified spines in posterior notopodia and sperm morphology in polydorins
FIGURE 14. Habitat of Dipolydora vietnamita sp. nov. (in life). A–C, worm burrows in a shell of the date mussel Lithophaga sp. which, in turn, have perforated the shell of an oyster. Arrow showing pygidium of a worm inside burrow. Scale bar: A–C = 1 mm.
FIGURE 12 in Three new species of shell-boring Dipolydora (Annelida: Spionidae: Polydorini) from the South China Sea and the Gulf of Thailand, Vietnam, with comments on the modified spines in posterior notopodia and sperm morphology in polydorins
FIGURE 12. Dipolydora vietnamita sp. nov. morphometric relationships. A, caruncle length and anterior position of gizzardlike structure versus total number of chaetigers. B, last branchiate chaetiger versus total number of chaetigers. Correlation coefficients (r) and their significance are reported in Table 1.
FIGURE 11 in Three new species of shell-boring Dipolydora (Annelida: Spionidae: Polydorini) from the South China Sea and the Gulf of Thailand, Vietnam, with comments on the modified spines in posterior notopodia and sperm morphology in polydorins
FIGURE 11. Morphology of Dipolydora vietnamita sp. nov. (formalin-fixed paratypes stained with methylene green). A, anterior end, dorsal view, showing unusual pattern of MG staning on anterior chaetigers. B, chaetigers 3–12, ventral view, showing transverse bands of MG staining on chaetigers 6–11. C, middle chaetigers, dorsal view. D, E, posterior end in left lateral (D) and rear (E) view. F, general dorsal view, showing common absence of MG staining on dorsal side of anterior chaetigers. G, anterior fragment, left lateral view, showing MG staining on peristomium, ventral side of chaetigers 6–11, and of glandular pouches in lateral side from chaetiger 12 onwards. H, chaetiger 5, right side in ventral view. Abbreviations: ch5, ch11 = chaetigers 5, 11; co = bilimbate companion chaetae; fa = heavy falcate spines; su = dorsal superior capillaries; ve = ventral capillaries. Scale bars: A–E = 100 µm; F, G = 200 µm, H = 20 µm. A, B, E, G, H—MIMB 42724; C, F—MIMB 42728; D—MIMB 42723.
FIGURE 10 in Three new species of shell-boring Dipolydora (Annelida: Spionidae: Polydorini) from the South China Sea and the Gulf of Thailand, Vietnam, with comments on the modified spines in posterior notopodia and sperm morphology in polydorins
FIGURE 10. Morphology of Dipolydora spinosa sp. nov. (formalin-fixed paratypes stained with methylene green). A, middle part of body from chaetiger 14 onwards, dorsal view, showing beginning of dorsal staining from chaetiger 15. B, middle chaetigers, dorsal view. C, anterior end, ventral view, showing stained transverse bands on chaetigers 7–13. D, anterior end, dorsal view (head in frontal view), showing MG staining of foregut. E, middle female chaetigers, dorsal view, showing dorsal terminal parts of nephridia. Abbreviations: ch5, ch15 = chaetigers 5, 15; ne = nephridium. Scale bars: A–E = 100 µm. A, C, E—MIMB 42714; B, D—MIMB 42719.
FIGURE 7 in Three new species of shell-boring Dipolydora (Annelida: Spionidae: Polydorini) from the South China Sea and the Gulf of Thailand, Vietnam, with comments on the modified spines in posterior notopodia and sperm morphology in polydorins
FIGURE 7. Morphology of Dipolydora spinosa sp. nov. A–D, anterior ends: A, left lateral view; B, D, ventral view; C, dorsal view. E–G, posterior ends: E, left lateral view; F, dorsal view; G, dorso-lateral view. H, notopodia of posterior chaetigers with capillary chaetae and awl-like spines. Abbreviation: an = occipital antenna, ch5 = chaetiger 5. Scale bars: A–D = 300 µm; E–G = 200 µm; H = 50 µm. A–F—holotype MIMB 42721, in life; G, H—paratype MIMB 42720, in life.
FIGURE 6 in Three new species of shell-boring Dipolydora (Annelida: Spionidae: Polydorini) from the South China Sea and the Gulf of Thailand, Vietnam, with comments on the modified spines in posterior notopodia and sperm morphology in polydorins
FIGURE 6. Dipolydora spinosa sp. nov. morphometric relationships. A, caruncle length and anterior position of gizzard-like structure versus total number of chaetigers. B, last branchiate chaetiger and anterior position of notopodial awl-like spines versus total number of chaetigers. Correlation coefficients (r) and their significance are reported in Table 1.
FIGURE 5 in Three new species of shell-boring Dipolydora (Annelida: Spionidae: Polydorini) from the South China Sea and the Gulf of Thailand, Vietnam, with comments on the modified spines in posterior notopodia and sperm morphology in polydorins
FIGURE 5. Morphology of Dipolydora spinosa sp. nov. (in life). A, B, anterior end, dorsal view. C, middle chaetigers, dorsal view, showing gizzard-like structure. D, gizzard-like structure, dorsal view, showing grain-like structures in the wall. Abbreviation: ch5 = chaetiger 5; gi = gizzard-lile structure. Scale bars: A–C = 200 µm; D = 50 µm. A, C, D—41-chaetiger individual; B—70-chaetiger individual. A–D—MIMB 42719.
FIGURE 3 in Three new species of shell-boring Dipolydora (Annelida: Spionidae: Polydorini) from the South China Sea and the Gulf of Thailand, Vietnam, with comments on the modified spines in posterior notopodia and sperm morphology in polydorins
FIGURE 3. Morphology of Dipolydora echinata sp. nov. (formalin-fixed holotype MIMB 42702 stained with methylene green). A, posterior end, dorsal view. B, notopodia of posterior chaetiger with thin protruding capillary chaetae and thick embedded awl-like spines. C, pygidium, dorsal view. Scale bars: A–C = 50 µm.
FIGURE 8 in Three new species of shell-boring Dipolydora (Annelida: Spionidae: Polydorini) from the South China Sea and the Gulf of Thailand, Vietnam, with comments on the modified spines in posterior notopodia and sperm morphology in polydorins
FIGURE 8. Morphology of Dipolydora spinosa sp. nov. A–F, chaetiger 5 chaetae: A, C, bilimbate companion chaetae; B, D, falcate spines in lateral view; E, ventral capillary chaeta; F, dorsal superior geniculate capillary chaeta. G, H, neurochaetae from chaetiger 7: G, inferior winged capillary chaeta; H, bidentate hooded hook. I, J, neurochaetae from a posterior most chaetiger: I, inferior hair-like capillary chaeta; J, bidentate hooded hook. K, L, notochaetae from a posterior most chaetiger: K, three awl-like spines with basal part blunt; L, capillary chaeta. M, octad of spermatids (one below is not visible). Scale bars: A–J, K, L = 20 µm; M = 5 µm. A–J—MIMB 42714; K–M—MIMB 42715.
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