Multiple Origins of Elytral Reticulation Modifications in the West Palearctic Agabus bipustulatus Complex (Coleoptera, Dytiscidae)

The Agabus bipustulatus complex includes one of Europe's most widely distributed and common diving beetles. This complex, which is known for its large morphological variation, has a complex demographic and altitudinal variation in elytral reticulation. The various depth of the reticulation imprint, both in smaller and larger meshes, results in both mat and shiny individuals, as well as intermediate forms. The West Palearctic lowland is inhabited by a sexually dimorphic form, with shiny males and mat females. In mountain regions, shiny individuals of both sexes are found intermixed with mat individuals or in pure populations in central and southern areas, whereas pure populations of mat individuals are exclusively found in the northern region at high altitude. Sexual selection is proposed as a driving force in shaping this variation. However, the occurrence of different types of reticulation in both sexes and disjunct geographical distribution patterns suggest an additional function of the reticulation. Here we investigate the phylogeographical history, genetic structure and reticulation variation of several named forms within the Agabus bipustulatus complex including A. nevadensis. The molecular analyses recognised several well-supported clades within the complex. Several of the named forms had two or more independent origins. Few south European populations were uniform in reticulation patterns, and the males were found to display large variation. Reticulation diversity and population genetic variability were clearly correlated to altitude, but no genetic differences were detected among populations with mixed or homogenous forms. Observed reduction in secondary reticulation in female and increased variance in male at high altitude in South Europe may be explained by the occurrence of an additional selective force, beside sexual selection. The combined effect of these selective processes is here demonstrated in an extreme case to generate isolation barriers between populations at high altitudes. Here we discuss this selective force in relation to thermal selection

Sampled species and collection sites along with their respective locality code.

<p>Collectors of the populations/specimens are: Anders Nilsson (AN), Bertil Andrén (BA), Carmen E. Sainz Cantero (CSC), Ignacio Ribera (IR), Johannes Bergsten (JB), Marcus K. Drotz (MD), Stefan Ericsson (SE), and Shidi O. Hosseinie (SH). Map codes are given for <i>A. bipustulatus</i> collection sites, which are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0009034#pone-0009034-g002" target="_blank">figure 2</a>. Collection sites in bold were used both in the morphological and population genetic studies.</p

<i>Agabus bipustulatus</i> collecting sites.

<p>Map codes and corresponding locality codes are given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0009034#pone-0009034-t001" target="_blank">table 1</a>.</p

Relationship between male secondary elytral reticulation diversity and altitude in <i>Agabus bipustulatus</i>.

<p>Reticulation diversity was estimated with the Simpson diversity index (<i>D</i>) based on the data given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0009034#pone-0009034-t003" target="_blank">table 3</a>. Low index value (1-<i>D</i>) indicates small level of variation in the secondary reticulation. Geographically close populations are connected with lines. Altitude is given in meters. Locality codes follow <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0009034#pone-0009034-t001" target="_blank">table 1</a>.</p

Morphological shape variation between and within <i>Colymbetes paykulli</i> males from the Palearctic (○) and Nearctic (■) regions.

<p>Shape variation was analysed with a Principle Component Analysis (PCA) of the partial warps, the uniform components and centroid size of the body in the Relative warps program v 1.20 (Rohlf 1998). The shape difference between the regions is significant (first component d.f. = 86, t-value = 13.30 p<0.001). The Nearctic <i>C</i>. <i>paykulli</i> is within this study accepted as a valid species and should be known as <i>C</i>. <i>longulus</i> LeConte.</p

Relationship between observed mean heterozygosity and estimated inbreeding over populations, and altitude in <i>Agabus bipustulatus</i>.

<p>Estimated (•) observed mean heterozygosity (<i>R²-adj</i> =  0.185, <i>P</i> =  0.033); (○) estimated inbreeding coefficient (<i>F</i>_IS) over populations (<i>R²-adj</i> =  0.168, <i>P</i> =  0.041). Altitude is given in meters.</p

Strict consensus tree from parsimony analysis of <i>Agabus bipustulatus</i> complex plus outgroups.

<p>The combined unweighted parsimony analysis includes all three genes (cytochrome b, cytochrome c oxidase subunit I and cytochrome oxidase II) and the outgroup species <i>Agabus nebulosus, A. affinis</i> and <i>A. guttatus</i>. Number of fundamental trees = 13. Bootstrap values above 50% are reported above branches. Bremer support values are reported below branches. Locality codes follow <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0009034#pone-0009034-t001" target="_blank">table 1</a>.</p

<i>Agabus bipustulatus</i> classification of secondary elytral reticulation after the total sample was screened for common patterns.

<p>Seven categories (A-G) ranging from tightly packed longitudinally elongated meshes to more or less isomorphic meshes were recognised in <i>Agabus bipustulatus</i>. Type A is found in the <i>bipustulatus</i> and <i>solieri</i> forms, type B in <i>kiesenwetterii</i>, <i>falcozi</i> and <i>dolomitanus</i>, and type C, F and G in <i>pyrenaeus</i>. The categories D, E and F describe intermixed patterns.</p