Water chemistry variation promotes adaptive radiation in
three-spined stickleback (Gasterosteus aculeatus)
Mahmuda Begum1, Victoria Nolan1 and
Andrew D. C. MacColl1
School of Life Sciences, University of Nottingham, Nottingham NG7 2RD,
United Kingdom.
Corresponding author
Mahmuda Begum
PhD fellow
School of Life Sciences,
University of Nottingham,
Nottingham NG7 2RD,
The United Kingdom
E-mail:
mahmudabegum.bcsir@gmail.com
Abstract
The context and cause of adaptive
radiations has been widely described and explored but why rapid
evolutionary diversification does not occur in related evolutionary
lineages has yet to be understood. One possible answer to this is simply
that evolutionary diversification is provoked by environmental
diversity, and that some lineages do not encounter the necessary
environmental diversity. Three-spined stickleback on the Scottish island
of North Uist show enormous diversification, which seems to be
associated with the diversity of aquatic habitats. Stickleback on the
neighbouring island of South Uist have not been reported to show the
same level of evolutionary diversity, despite levels of environmental
variation that we might expect to be similar to North Uist. In this
study, we compared patterns of morphological and environmental diversity
on North and South Uist. Ancestral anadromous stickleback from both
islands exhibited similar morphology including size and bony ‘armour’.
Resident stickleback showed significant variation in armour traits in
relation to pH of water. However, North Uist stickleback exhibited
greater diversity of morphological traits than South Uist and this was
associated with greater diversity in pH of the waters of lochs on North
Uist. Highly acidic and highly alkaline freshwater habitats are missing,
or uncommon, on South Uist. Thus, pH appears to act as a causal factor
driving the evolutionary diversification of stickleback in local
adaptation in North and South Uist. This is consistent with
diversification being more associated with ecological constraint than
ecological opportunity.
Keywords : Stickleback, morphometric study, North Uist, South
Uist.
Studies of adaptive radiation have tended to concentrate, for obvious
reasons, on lineages and locations in which it is known to have
occurred. The circumstances associated with a failure to radiate are
less well explored, yet there is much that might be learned from them
about the conditions that favour divergence. Differences in ecological
conditions among local populations are generally assumed to be the
reason for adaptive divergence leading to adaptive radiation,
reproductive isolation and speciation (Schluter 1996; Schluter 2000;
MacColl 2011). In particular, according to the ecological theory of
adaptive radiation (Schluter, 2000), ‘ecological opportunity’, the
diversity of available resources not used by other taxa, is central to
explaining why adaptive radiations occur. Lineages often diversify when
they colonise habitats where there is little competition for
constraining resources (Simpson 1953, Schluter 2000). The concentration
on biotic interactions, especially competition for food, that has
followed from the idea of ecological opportunity, has meant that the
role of abiotic environmental variation has been less well explored.
Here, we scrutinise diversification of three-spined stickleback
(Gasterosteus aculeatus ) in lochs on two neighbouring Scottish
islands, for clues about the causes of adaptive radiation.
The three-spined stickleback (hereafter ‘stickleback’) has served as a
model organism for the study of adaptive radiation due in part to
parallel diversifications of freshwater populations from marine
ancestors, that have occurred in the last 10,000 years ago (Bell and
Foster 1994; Schluter 2000; Jones et al 2012; Magalhaes et al. 2021).
Freshwater populations have evolved conspicuous differences in
morphology, physiology and behaviour (Bell and Foster 1994;
Ostlund-Nilsson et al. 2006). The divergence of morphological traits
through major changes in the bony armour that have repeatedly evolved in
various locations, are common
components of adaptive radiation of stickleback (Colosimo et al. 2004;
Chan et al. 2010). Ancestral marine anadromous sticklebacks are heavily
armoured with a continuous row of 30-36 bony lateral plates running from
head to tail on each side (known as a complete morph), and have dorsal
spines, a well-developed pelvic girdle and further spines attached to
the girdle (Colosimo et al. 2005; Barrett et al. 2008). On the other
hand, derived freshwater (live in freshwater year round) and
saltwater-resident (inhabit coastal saltwater all year round without
migration to the open sea) stickleback exhibit substantial reduction in
the total plate number with either a discontinuous row of 9-28 plates
(partial morph) or with 0-9 lateral plates at the anterior end (low
morph), and reductions in the size of spines and girdle (Colosimo et al.
2004; Colosimo et al. 2005). In addition, other phenotypic traits such
as body size and shape of freshwater and saltwater-resident stickleback
show morphological transformation from the ancestral anadromous form
(Moodie and Reimchen 1976; Schluter 1993; Bell et al. 1993; Schluter et
al. 2004; Shapiro et al. 2004; Ravinet et al. 2015).
To date, several abiotic and biotic environmental factors have been
considered as causes for these major phenotypic changes including
variations in salinity, temperature, nutrient and calcium availability,
stream gradient, predators and parasites (Giles 1983; Bergstrom 2002;
Barrett et al. 2009; Myhre and Klepaker 2009; Marchinko 2009; Wark and
Peichel 2010). For example, predation-driven selection has been reported
to influence the evolution of bony armour structures such as the lateral
plates and spines within marine and freshwater stickleback populations
(Marchinko 2009). Moreover, other factors such as reduced nutrients or
salinity, and calcium ion deficiency have also been reported to have an
association with the loss of lateral plates in freshwater populations
(Giles 1983; Barrett et al. 2009; Myhre and Klepaker 2009).
The neighbouring islands of North and South Uist, in the Scottish
Western Isles, appear to have much in common from the perspective of
aquatic habitats and species. Both have large numbers of small to
medium-sized shallow lochs, that occur over a variety of surface
geology, from peat and bedrock on their eastern sides, to shell-rich
machair sand on their western sides. Although fish, including
three-spined stickleback, populations have been surveyed on both islands
(Campbell and Williamson, 1979; Campbell, 1985), detailed studies of
phenotypic variation in the stickleback have only taken place on North
Uist (Giles 1983; MacColl et al. 2013; Magalhaes et al. 2016; Magalhaes
et al. 2021) apparently because of a lack of such variation on South
Uist (Campbell 1985).
North Uist comprises a mosaic of interconnected freshwater and brackish
water lochs and lagoons which are known to have exceptional variation in
their water chemistry, with high pH (~8) in the western
machair lochs in the west and low pH (~6) in the east.
These are associated with variation in the concentrations of alkaline
metals (sodium, potassium, magnesium, calcium etc.) (Waterston et
al.1979). This variation has been shown to correlate with the evolution
of high diversity of stickleback populations across the island (Giles
1983; MacColl et al. 2013; Magalhaes et al. 2016; Haenel et al. 2019)
and offers a unique opportunity to study the variation in phenotypes in
relation to adaptation under different environmental conditions.
In contrast, environmental conditions and phenotypic variation have been
poorly explored in South Uist, and there have been no reports of unusual
morphological variation in stickleback. While different in topography
(it has higher hills and less low-lying ground), it appears to have
similar variation in aquatic environments, with both saline and
freshwater lochs, and both western, machair lochs and eastern lochs on
peat or bedrock. This begs the question why, on two such closely
neighbouring islands, where ancestral variation is likely to be shared,
and environmental variation is similar, there appears to have been
little or no diversification of stickleback on one, while the other
exhibits some of the most dramatic variation known in the species. Here
we make a direct comparison of the morphological and environmental
variation in the two islands, and relate the former to the latter, to
understand what factors may promote or inhibit adaptive radiation at
local scales. We hypothesised that morphological traits of three-spined
stickleback would show variation between South Uist and North Uist due
to differences in the environmental diversity of the lochs.
Materials
and methods
2.1
Study area
Two neighbouring islands with
apparently similar aquatic environments were selected to make a
comparative study of morphological variation in three-spined
stickleback. North Uist (57°31’12”N; 7°27’42”W) is in the centre of the
Western Isles of Scotland and is approximately 303 km2in total area (Fig. 1B). It comprises a mosaic of peat bogs, heathland
and low hills which makes it different from South Uist. South Uist
(57°13’54”N; 7°02’38”W) is the second largest island of the Outer
Hebrides (Fig. 1C). It is around 320 km2 in total area
and differs greatly between its west and east sides.
2.2 Sample collection
Stickleback samples were
collected from 10 lochs on North Uist and 8 lochs on South Uist between
the dates of 06th – 16th May 2019
(Fig. 1). Locations on both islands were selected to maximise likely
variation in surface geology (sand versus peat or bedrock), and hence
water chemistry, using Google Earth. Stickleback were caught in
unbaited Gee’s Minnow Traps (Gee traps, Dynamic Aqua, Vancouver,
Canada), set overnight (approximately 16 hrs.) at all sites. The
geolocation of each sample site (GPSmap 60CSx, Garmin, UK) and water
quality parameters including temperature, absolute conductivity,
salinity and pH (multi-parameter probe - Multi340/set, WTW, Germany)
were recorded from all sites (Table A1 - A2). Lochs were grouped based
on the pH of water: high pH
>7.5, neutral pH = >7.0 - 7.4, low
pH< 7.0 (Table A1 - A2).
Across all ten sampling sites of North Uist, 135 stickleback were
haphazardly selected (Fig. 1B). From South Uist, 128 fish were collected
in the same way from 11 sites (3 sites from loch Aroa) in eight lochs
(Fig. 1C). Of the total 263 stickleback, 45 were anadromous fish sampled
from both islands to estimate the ancestral state of stickleback in the
Uist lochs. Fish were euthanized and preserved in 70% ethanol for
morphometric study.
2.3.Body armour and spine
data collection for morphometric analysis
To collect data on external bony skeletal structures (armour),
ethanol-preserved samples were stained with Alizarin Red solution
following a standard staining method (Peichel et al. 2001). Samples were
stored in 40% isopropyl alcohol (propan-2-ol). After confirming the
appearance of bony parts, a digital photograph of the right side of
every fish was captured using a digital SLR camera (Nikon D5200) with 60
mm macro lens, digital ring flash and a tripod (to set a fixed
distance). All photographs included graph paper as a scale (cm) and the
measurements of standard length (from the tip of the snout to the end of
caudal peduncle), first and second dorsal spine, pelvic spine (from
insertion point to the tip), pelvis height and pelvis length were
recorded using tpsDig2 version 2.31 (Rohlf 2010) (Fig. Appendix, A1).
The total number of lateral plates was also counted from the right side
of the stained photograph.
2.4 Data analysis
Data were collated in Excel
(Microsoft) and statistical analyses were conducted using R, version
3.6.3 (R Core Team 2020).