Dr Rudiger Riesch

Personal profile

Research interests

Research in my lab aims to understand the evolutionary processes that generate biological diversity. Using integrative approaches we try to answer the questions of (a) how and why organisms diversify phenotypically, (b) what reproductive barriers are important in facilitating a reduction in gene flow between diverging populations, (c) what evolutionary forces shape these reproductive barriers, (d) what role does human-induced environmental change have on biodiversity in general, and the stability of population differentiation among diverging populations in particular, and (e) why are some species so potent invaders? 

 

 

(1) Evolutionary Ecology of invasive poeciliid fishes 

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. 

 

 

(2) 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. 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 phenotypes are associated with exposure to stressors, (ii) what adaptations enable these fishes to cope with the different environmental stressors, (iii) to what degree is trait divergence in extreme habitats due to gene-by-environment interactions (i.e., phenotypic plasticity) versus heritable differences between populations, (iv) have the same divergent traits evolved independently along similar environmental gradients, and (v) does population divergence/local adaptation always lead to reproductive isolation from conspecifics inhabiting adjacent benign habitats. 

 

 

 (3) 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. In collaboration with Patrik Nosil from CEFE-CNRS in Montpellier,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.

 

 

(4) Maternal provision in livebearing 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?

 

 

(5) 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 hubbsiGambusia 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. 

 

 

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

Although generally regarded as a single species, numerous divergent killer whale lineages are recognized worldwide, which diverged in diet, behaviour, morphology, genetics, pigmentation, and social structure (Riesch et al. 2012). From 2000 to 2011 my research focused on acoustic communication and divergence of communication signals in killer whales from the eastern North Pacific. Here, 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.

  

 

(7) Coexistence in a bisexual/unisexual vertebrate mating system

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. In collaboration with Ingo Schlupp from the University of Oklahoma, our previous research focused on life-history differences between the species and how male mate choice might translate into potential life-history differences. 

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