Riesch Lab at RHUL

Our research focuses on understanding the evolutionary processes that generate biological diversity. In particular, we are interested in speciation as a result of divergent natural selection between ecologically different environments (ecological speciation). To that end, we combine behavioural, population genetic, life-history, and morphological techniques that include observational studies, manipulations in the field, and laboratory experiments.

Two main questions dominate our research program:

(i) How and why do organisms diversify phenotypically?

(ii) How and why do reproductive barriers evolve between diverging populations?

We put great emphasis on comparative analyses among species and populations, and make use of different study systems (livebearing fishes, insects and mammals) that span a range of biological diversity. Some of our main study systems are listed below.

(1) Evolutionary ecology of invasive poeciliids

Invasive alien species pose a direct threat to ecosystem health and stability, and are a major source of biodiversity loss. For example, they may fare better than native species in a warming climate, and often seem to outcompete native species for resources. European freshwater systems are of particular concern as, despite preventative legislation and initiatives, the number of established invasive species/populations continues to increase. This begs the question of which traits are important in facilitating these patterns.

Originally native to the Americas, livebearing fishes (family Poeciliidae) have been widely introduced as mosquito control agents or as a side effect of the aquarium trade. However, while as invasive alien species they have had strong negative effects on native biodiversity, effects on mosquitoes seem to be largely non-existent. Using invasive poeciliid fishes (including invasive guppies and mosquitofish) in Europe, China, Africa and South America, we are investigating the mechanisms behind their invasive success. Specifically, our work focuses on the role of behaviours, diet, life histories, morphologies, parasites, and genetics.

Patterns and drivers of multiple paternity we discovered across two species of invasive mosquitofish and across Italy, Spain and China (Gao et al. 2019).

Invasive Gambusia holbrooki in a small body of water in the Riu Ter Basin in Catalonia, Spain

Running boldness trials with invasive G. holbrooki in Spain

(2) Population divergence and ecological speciation in extremophile poeciliids

Extreme habitats are characterized by the presence of physiochemical stressors that require costly adaptations of any organism tolerating them. The presence of such stressors creates complex environmental gradients and affects both the ecology of individuals and the evolutionary trajectory of populations living along the gradients. In our lab, we work on several different of these stressors, including permanent darkness (as experienced in subterranean habitats), oil and industrial pollution, and hydrogen sulfide (H₂S) toxicity.

Our particular interest in these systems is on the role of life histories and behaviours in population divergence, reproductive isolations, and ultimately speciation. Specifically, we are investigating (i) what adaptations enable these fishes to cope with the different environmental stressors, (ii) to what degree is trait divergence in extreme habitats due to gene-by-environment interactions (i.e., phenotypic plasticity) versus heritable differences between populations, (iii) whether the same divergent traits evolved independently along similar environmental gradients, and (iv) whether population divergence/local adaptation always leads to reproductive isolation from conspecifics inhabiting adjacent benign habitats.

Cloudy (yellow-greenish) water downstream of sulfur springs is due to suspended colloidal sulfur.

An oil-polluted guppy habitat near Point Fortin on Trinidad; photo by F. Santi

Fecundity as a function of H₂S concentration. This drastic reduction in fecundity is usually coupled with an increase in offspring size as evidenced by the pictures of ready-to-be-born embryos of Gambusia sexradiata (left) from a non-toxic habitat vs. G. eurystoma (right) from the highly toxic Bañõs del Azufre.

(3) Population divergence and ecological speciation in Gambusia spp. from The Bahamas

Several different species of mosquitofish (genus Gambusia) inhabit the different islands of the Bahamas archipelago. Our research in The Bahamas has mainly focused on two different kinds of habitats, tidal creeks and blue holes.

Tidal creeks are shallow estuaries influenced by the tides. Starting in the late '50s, a large number of roads have been constructed across The Bahamas, often fragmenting these tidal creeks. This results in dramatically reduced tidal exchange in these fragmented sections of the tidal creeks, usually resulting in increased sedimentation rates, greater oxygen and temperature extremes, and changes in ecosystem composition. In this sytem, we are investigating rapid human-induced life-history evolution in Bahamas mosquitofish (Gambusia hubbsi, Gambusia manni and an as-yet undescribed species, Gambusia sp.) inhabiting fragmented and unfragmented tidal creeks on six different Bahamian islands.

Inland blue holes are essentially water-filled, vertical caves that are surrounded by a sea of land. These blue holes are widespread across Andros Island, and are often inhabited by Gambusia hubbsi. In some of these blue holes G. hubbsi experience relatively predator-free environments that are devoid of piscivorous fishes; in others G. hubbsi are heavily preyed upon by the bigmouth sleeper (Gobiomorus dormitor). Our research in this system mainly focuses on the relative importance of predation regime compared to other environmental factors (e.g., population density, primary productivity, or sex ratio) on body-shape and life-history trait divergence in G. hubbsi within and between predator regimes.

Male life-history differentiation in Bahamas mosquitofish from 7 high-predation (HP, red) and 7 low-predation (LP, blue) blue holes.

Female life-history differentiation in Bahamas mosquitofish from 7 high-predation (HP, red) and 7 low-predation (LP, blue) blue holes.

(4) Ecological speciation in Timema walking-stick insects

Pathways that can lead to reproductive isolation can be categorized as occurring either before or after the formation of hybrid zygotes. Current evidence suggests that pre-zygotic isolation evolves faster than intrinsic post-zygotic isolation, and one important way in which pre-zygotic isolation may evolve is via reduced heterotypic mating due to divergence of mate preferences and traits (= sexual isolation). For example, it has been suggested that adaptation to local environments can lead to population divergence in communication signals (= sensory drive), thereby incidentally resulting in sexual isolation. However, despite the accumulation of examples of sexual isolation, for most study systems there are still fundamental, unresolved questions concerning which traits sexual isolation is actually focused upon, and what their underlying genetic basis is.

The genus Timema (Phasmatodea: Timemidae) consists of around 20 species of small, wingless, and plant-feeding walking-stick insects that are common throughout Southwestern North America (i.e., primarily California). Timema spp. present an outstanding model system for investigating the traits and genes involved in speciation, because they exhibit highly variable levels of sexual isolation between and within species. While sexual isolation has been documented between certain host-plant ecotypes (i.e., populations within species feeding on a particular species of host plant), the actual traits or sensory modalities involved in establishing sexual isolation have yet to be characterized. Our research on Timema spp. employs an integrated ecological, experimental, and genomic approach to investigate the role of chemical communication (via cuticular hydrocarbons) as a mechanism of sexual isolation in the group.

Female Timema podura on chamise (Adenostoma fasciculatum)

Male Timema poppensis on coast redwood (Sequoia sempervirens)

(5) Maternal provisioning strategies in poeciliid fishes

The full range of maternal provisioning can be envisioned as a continuum. On one end of the continuum, all nutrients necessary for full embryonic development are packaged into the yolk prior to fertilization (i.e., lecithotrophy), while the developing embryo relies almost exclusively on a continuous supply of nutrients obtained directly from the mother during gestation (i.e., matrotrophy) on the other end of the continuum. According to the Trexler-DeAngelis model of maternal provisioning (Trexler & DeAngelis 2003), postfertilization maternal provisioning is most likely to evolve in environments with constant, high levels of resource availability.

Female livebearing fishes (Poeciliidae) utilize the full spectrum of this continuum to provision the young that develop within their reproductive tract. While the general strategy is largely species-specific, substantial variation also exists between different populations within a species. Drawing on the diverse livebearing fish systems we work on, we try to address the question of what ecological factors might influence which specific strategy is employed by females of a population at any given time?

The continuum of maternal provisioning ranging from lecithotrophy via incipient matrotrophy to substantial matrotrophy as visualized by the Matrotrophy Index (MI), which tracks embryo weight during gestation. If the eggs were fully provisioned by yolk before fertilization (i.e., lecithotrophy) then we would expect the embryos to lose 25%-40% of their dry mass during development (MI between 0.60 and 0.75).

(6) Population divergence in killer whales (Orcinus orca)

(Currently no active research)

Although generally regarded as a single species, numerous divergent killer whale (Orcinus orca) lineages are recognized worldwide that diverged in diet, behaviour, morphology, genetics, pigmentation, and social structure. In the eastern North Pacific, three sympatric ecotypes (residents, transients, and offshores) coexist in sympatry. Resident killer whales specialize on fish (salmon in particular), transients hunt marine mammals, and offshores probably also specialize on fish (albeit species like Pacific sleeper sharks and Pacific halibut).

Killer whales produce three types of sounds: echolocation clicks are thought to function in orientation and prey detection, whereas pulsed calls and whistles are communicative signals. Despite evidence for some universal acoustic signals, the structure and frequency of use of most vocalizations differs strikingly between ecotypes. Evidence suggests that these differences are partially due to differences in hearing sensitivities of killer whale prey: marine mammals have good underwater hearing and exhibit anti-predator behavior in response to transient (i.e., marine mammal-eating) killer whale vocalizations. Salmon and other fishes, on the other hand, cannot detect killer whale sounds over significant distances.

Our previous research mainly focused on whistle communication. Killer whale whistles are highly modulated signals that show some degree of directionality and have lower sound pressure levels and higher fundamental frequencies compared to pulsed calls. A large proportion of a killer whale’s whistle repertoire is made up of stereotyped whistles that are often emitted in elaborate sequences. Furthermore, whistle types of northern residents, southern residents, and transient killer whales differ drastically in structure, and while residents whistle during almost all behavioural contexts (albeit most often during socializing), transients restrict whistling to non-foraging situations.

Male northern resident killer whale surfacing in our wake waves.

Male northern resident killer whale diving.

(7) Coexistence in a bisexual/unisexual mating system

(Currently no active research)

Among unisexual vertebrates, modern bony fishes (Teleostei) are of central interest because different modes of asexual reproduction have repeatedly evolved within this group: besides hemiclonal inheritance (hybridogenesis) and facultative parthenogenesis, several unisexual fishes are known to reproduce via sperm-dependent parthenogenesis (or gynogenesis), in which sperm is required to initiate embryogenesis, even though oocytes do not undergo meiosis, and inheritance is strictly maternal.

The gynogenetic, all-female Amazon molly (Poecilia formosa), for example, resulted from a single hybridization of the two bisexual species Poecilia latipinna and Poecilia mexicana, and predominantly uses sperm from males of these two species for gynogenetic reproduction. This, however, creates a paradox: If all else is equal, unisexuals produce twice as many daughters (provided a 1:1 sex ratio in the bisexual species) and should quickly outcompete bisexuals, thereby driving ecologically similar bisexual taxa to extinction – which in turn would lead inevitably to their own extinction. Hence, the mechanisms underlying maintenance of coexistence are of considerable interest in evolutionary ecology.

Several hypotheses have been put forth to explain the apparent coexistence of Amazon mollies with their hosts, among them the frozen niche variation hypothesis (for details please refer to the publications by Robert Vrijenhoek), the Red Queen hypothesis (for details see, e.g., this publication), the behavioural regulation hypothesis, and the life-history regulation hypothesis. We are particularly interested in the interplay between the latter two. The behavioural regulation hypothesis basically posits that male mate choice plays an important role in regulating the mating complex, and that unisexual females may not receive sufficient sperm from ‘host’ males to continuously fertilize all of their oocytes. The life-history regulation hypothesis, on the other hand, essentially predicts coexistence if lifetime reproductive success of unisexuals was half that of their sexual counterparts. Our previous research questions focused on life-history differences between the species and how male mate choice might translate into potential life-history differences.

The Amazon molly (Poecilia formosa); photo by M. Tobler

One of our Amazon molly field sites in Central Texas

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