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VARIABILITY IN LAND SNAILS 817

(Shimizu & Ueshima, 2000; Davison, 2002; Teshima According to autocorrelation statistics for shell
et al., 2003). shape similarity (Fig. 5A, B), no spatial structure for
Similar events may also be described for Sicilian all western and central populations (0–125 km) is
Marmorana: there was a morphological gradient of illustrated by fluctuations around 0 values. Notably,
characters related to globularity (shell shape index the analysis confirmed patchy shell shape distribu-
ShH/MaxD) (Fig. 3); laboratory studies have shown tion (flat to globular) in this geographic area. The
that flat keeled and globular snails mate and produce sudden increase in significant positive values over
viable offspring (Rensch, 1937) and that the different the scale of 125 km is in line with the observation
populations have similar genitalia. Although the that, along the Sicilian northern mountain chain
present data do not allow us to confirm the hypothesis (Madonie Mountains), only globular shapes are
of introgression by hybridization, it is noteworthy present and cluster in a monophyletic assemblage
that intermediate shapes are present where extreme (clade 3). The sharp drop to significant negative
shapes (globular and flat) are in close contact. Thus, values, over the scale of 200 km, for shell shape and
the co-presence of globular, intermediate and flat mtDNA sequence correlograms, is in line with the
shells in single lineages could be a consequence of a large gap separating the limestone Iblean plateau
parapatric pattern of populations and/or of a process and the northern and western side of the calcareous
of speciation in re. It is noteworthy that the popula- Sicilian northern mountain chain. Moreover, with
tion distribution observed in the field over large areas respect to the mtDNA sequence correlogram, geo-
of Sicily effectively confirmed a parapatric pattern of graphic isolation events are illustrated by highly
distribution. positive values for very close neighbouring popula-
Regarding genitalia, changes as sharp as those tions (0–25 km), abruptly decreasing to lower
observed in shells were not observed. Sicilian Mar- values for more distant populations (50 km). Hence,
morana show a multiple scale variation pattern the spatial pattern of distance shows that highly
(Fiorentino et al., 2008) consistent with the scenarios morphologically differentiated populations are
proposed in other studies: genital variance may be genetically close when geographically close. The
selectively controlled to maintain function (Baminger progressive decline over the approximate distance
& Haase, 2000), such as strong control of the number range 0–250 km is consistent with a model of isola-
of allosperm (Lind, 1973; Haase & Baur, 1995) or tion by distance, a pattern already recognized in
differences in the number of sperm transferred other groups of land snails (Madec et al., 2003). In
(Locher & Baur, 1997; Baur, Locher & Baur, 1998). a group largely affected by limited and aggregated
However, DFA and ANOVA significantly distinguished distribution due to low vagility, dispersion capacity,
MP specimens (Monte Pellegrino), a lineage signifi- and obligatory constraint to calcareous rock habi-
cantly distinct and diversified in the phylogram tats, vicariance and isolation by distance therefore
(Fig. 6B). presumably played a key role in diversification pro-
cesses. Plio-Pleistocene paleogeographic changes
(involving sea level changes and leading to the
PHYLOGENETIC RELATIONSHIPS current geomorphological setting of Sicily) and
CO I and 16S sequence data suggest that the mor- overall postglacial land expansion during the Pleis-
phological characters used in the taxonomy of the tocene (Agnesi et al., 2000) may have created isola-
Sicilian Marmorana to define the five recognized tion by distance and may both have played major
species do not reflect phylogenetic relationships. roles in establishing the present spatial structure of
Indeed, mitochondrial based reconstructions (Fig. 6B) genetic and morphological variation in this area of
identify several supported subclusters not related to distribution.
shell shape but well correlated with geographic dis- Basal relationships are not sufficiently resolved in
tribution (Fig. 6A). For example, in central-eastern the analysis for the amplified fragments to provide
and eastern Sicily, where both shell shapes are wide- sufficient information to detect relationships at this
spread, inferred topologies group together globular, level. Increasing the number of characters may lessen
flat and intermediate shapes (clade 2 and 4). By such problems, assuming similar evolutionary pat-
contrast, the phylogenetic assemblage coherent with terns of additional nucleotides (Berbee, Carmean &
shell morphotypes is represented by the cluster Winka, 2000). On the other hand, if the observed lack
including populations of the Sicilian northern moun- of phylogenetic resolution reflects rapid radiation, no
tain chain (Madonie Mountains), a geographic area in amount of data would improve resolution. The
which only globular shapes are found (clade 3). The general understanding of phylogenetic relationships
only exceptions to this phylogeographic pattern are in the genus Marmorana, acquired in the present
the ‘M. muralis’ cluster (discussed below) and the study, suggests that further study with nuclear
unresolved lineages of Caccamo and San Fratello. markers would be worthwhile.

© 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 94, 809–823

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