IntroductionSpeciation is a continuum toreproductive isolation. It is driven by little adaptations resulting in populationdivergence.
Divergence in sensory properties (e.g. vision) can have a profoundeffect on speciation, as it not only affects ecological performance, but alsosexual communication. Fish are often used in sensory divergence studies, asthey exhibit strong sensory drive in visual communication. Yet, a causal link betweendivergent visual adaptation and reproductive isolation has not been found. State of the artAccording to the sensory drivehypothesis, sensory signals, systems and communication behaviour adapt with thesensory environment. This suggests that environmentally conspicuous signalswill be sexually preferred.
There is only one known study to experimentallyalter visual conditions during developmental stages to test the effect ofenvironmental visual adaptations on mate choice. Fuller & Noa (2010) didnot find an overall effect on female mating preference for different coloured killifish(Fundulidae). Though, interaction between genetic background and lightconditions suggests that visual development did influence colour preference.
Togain further understanding of this influence Wright et al. (2017) used twoclosely related African Pundamilia cichlids,both sexually dichromatists. Males differ in colour, with Pundamilia pundamilia being blue and Pundamilia nyererei being red, while females of both species have ayellow/grey colouration. P. pundamilia andP. nyererei live sympatric insouthern Lake Victoria, East Africa. They are known to hybridize in turbidwaters, whereas they behave as biological species in clearer waters.
Eventhough these rock dwelling cichlids co-occur, they tend to reside at differentdepths. P. nyererei resides atgreater depths than P. pundamilia,which can be found in the shallower waters of the lake. In the turbid LakeVictoria, depth comes with a shift in light spectrum. Short wave length lightdoes not penetrate to the habitat of P.nyererei. This makes the habitat of P.
nyererei devoid of blue light. Carleton et al. (2005) demonstrated visualsystem differences between the species. P.nyererei was found to have a higher level of long-wavelength-sensitive(LWS) visual pigment expression. Additionally, it was found to be more redsensitive as a result of a LWS allele it carries. According to findings of Maanet al.
(2006) P. nyererei also isbehavioural more sensitive to long-wavelength light than P. pundamilia is. It remains unclear whether these differences invisual system cause differences in mate preference, possibly drivingspeciation. Recent findingsTo study the mechanistic linkbetween colour vision and mate preference, variation in visual perception mustbe isolated and manipulated. Wright et al. (2017) manipulated visual perceptionby rearing the species in light conditions simulating either shallow or deeperwaters of Lake Victoria.
By assessing their mate choice in both their naturaland unnatural light conditions the effect of developmental plasticity in thevisual system on behavioural mate choice was investigated. Male courtshipbehaviour was divided in lateral display (LD) and quiver (Q), scoring them separatelyfor female preference. Two types of set-ups were used, one of them allowingchemical communication, which is known to have no effect on female matepreference. Difference between the set-ups were nonsignificant, but chemicalcommunication increased female activity.
Lateral displaying of males resultedmore frequently in a positive response by the female. Quivering males did nothave a different preference across the two set-ups. Wright et al.
(2017)hypothesised deep-reared females to prefer red males (P. nyererei) under both light conditions, as red is equallyconspicuous in both environments. Shallow-reared females were expected to onlyprefer blue males (P. pundamilia) ina shallow-light environment. Rearing light was found to significantly influencemate choice for both courtship behaviours. P.pundamilia had most positive responses from shallow reared females, whereasdeep-reared females had no significant preference.
Wright et al. (2017) also foundhigher female responsiveness to conspecific males when reared under their naturallight conditions. P. pundamilia significantlyfavoured conspecific males in their natural shallow light condition. However, P. nyererei did not prefer conspecificmales, neither in shallow nor in deep-water light conditions. They found anon-significant tendency for species-assortative preference when females were rearedin natural light conditions, but not for females reared in unnatural situations.Species-assortative preference for LD males was non-significantly affected by rearingenvironment.
Quivering preference on the other hand was significantlyinfluenced. These findings show that light conditions not only influence matepreference, but it also affects species assortment. Discussion and future developmentsResults found by Wright et al.
(2017) do suggest that rearing environment affects mate preference in Pundamilia cichilds. This might be aresult of plasticity in visual properties in cichlid fish, which can be influencedby light. This plasticity may have resulted in deep-reared females being morered-sensitive, while the shallow-reared fish may have gained a greatersensitivity to blue colouration. However, the data found suggests thatvariation in visual perception probably was not the sole determinant of femalepreference, as female preference did not show a strong interaction between rearinglight and test light. An alternative explanation, suggested by Wright et al.(2017), concerns female exposure to male phenotypes prior to testing.
Femalesreared in different light conditions have different experiences with the male phenotypes.Another factor that cannot be rules out is possible imprinting on brother phenotypes.Another apparent result found was the low species-assortative preference, evenwhen reared and tested in their natural light conditions. Given that previous studiesreported conspecific preference in both species, this is an unexpected outcome.
The experimental light conditions, the lack of maternal imprinting andselective disappearance are explanations suggested by Wright et al. (2017). Testedfish were only exposed to brothers and sisters, allowing for filial imprinting withoutmaternal imprinting possible.
However, Verzijden et al. (2008) has shown thatmaternal imprinting may be a strong determinant of female mate choice in LakeVictoria cichlid fish. The weak species-assortative preference might also becaused by selective survival of the species. Survival might have been non-random,with increased mortality in unnaturally reared fish, which might have led to a non-randomtest population. These underlying mechanisms are in need of further investigation.
Plasticity in visual development and phenotypical or chemical imprinting areall potential subjects for further investigation. Overall, light conditions significantlyinfluence female preference, highlighting the importance of heterogenicenvironments in the process of speciation.