Scale-eating cichlids: from hand(ed) to mouth

Two recent studies in BMC Biology and Evolution raise important questions about a textbook case of frequency-dependent selection in scale-eating cichlid fishes. They also suggest a fascinating new line of research testing the effects of handed behavior on morphological asymmetry. See research article http://www.biomedcentral.com/1741-7007/8/8.

Among six species of scale-eating cichlids, including P. microlepis, geometric morphometric measures of overall head-shape asymmetry (when viewed laterally) were only signifi cant in adults of one of the two most derived species, P. straeleni [2]. Measurements of lower jaw mechanical advantage diff ered between right-and left-bending forms in the two most derived scale-eaters, but these diff erences were only marginally signifi cant for three of the four tests. So, although the four most derived species eat scales almost exclusively, the statistical signal for asymmetry in the heads and jaws of adults appears surprisingly weak.
Similarly, when viewed from above, the mouth-bending angle also appears to be less discrete than generally believed [3]. Careful head-orientation measures of live, adult P. microlepis confi rmed that some bent towards the right and others to the left. But the frequency distribution of asymmetry values was only weakly bimodal, with modes corresponding to bending angles of 2-3 degrees. Here again, right and left bending did not appear to be discrete states in adults, and more then 10% of individuals were nearly symmetrical (mouth bend less than 1 degree). As Van Dooren et al. note "mouth bend asymmetry is diffi cult to determine by visual inspection alone" [3].
Finally, temporal variation in the proportions of leftand right-bending P. microlepis is much less pronounced than generally believed. Unlike for total population size, where a confi dence limit is diffi cult to estimate, confidence limits to the proportions of right-and left-bending individuals within a sample are easily obtained from a binomial distribution. Of 13 samples taken over an 11-year period (see Figure 2A of [1]), only two approached statistical signifi cance. Furthermore, in all three populations where two samples were taken concurrently, one yielded an excess of left-bending individuals and the other a defi cit. In other words, variation among samples, and therefore among years, was not signifi cantly greater than expected due to simple sampling error.
Mouth-bending is even less pronounced in juvenile P. microlepis. Th e quantitative analyses of head-bending variation among brooded off spring from a single female [2] yielded two puzzling observations. First, "only 93 (of 141) animals showed unambiguous jaw laterality". Second, although some individuals departed from symmetry by several degrees, the distribution of jaw bending angles was clearly unimodal and centered on zero (see Figure 4C of [2]). If two alleles at a single locus induce discrete right-and left-bending morphs [1,6], then two questions arise. Why do Stewart and Albertson [2] fi nd that so many juveniles (close to one-third) exhibit no signifi cant mouth bending? And why does asymmetry in juveniles not materialize as a bimodal distribution of right-left diff erences?

The genetics of mouth bending
To complicate matters further, the genetic basis of mouth-bending has become more rather than less puzzling as more data accumulate. Two alleles, R and L, at a single locus are thought to direct bending to the right or the left, respectively [1,6]. But two confl icting models have been advanced. Th e fi rst is a simple model where R is dominant [1] ( Additional puzzling results emerge from two genetic marker studies. In the fi rst, Stewart and Albertson [2] measured mouth asymmetry quantitatively in juveniles and genotyped them for two microsatellite markers, A and B, putatively linked to right-bending (R) and leftbending (L) alleles, respectively. Th eir results are not consistent with either of the proposed genetic models. First, nearly half of left-bending juveniles (20 of 45) carried marker A, a result inconsistent with the dominance of allele R. Second, nearly 10% of individuals (9 of 93), regardless of mouth bend, were homozygous for marker A, a result inconsistent with RR lethality (Figure 2, Model II). Th ird, over 12% of right-bending juveniles (6 of 48) were homozygous for marker B, a result inconsistent with being homozygous recessive for allele L. Finally, although both parents of this brood were inferred to be heterozygotes, and other microsatellite evidence pointed to a single male parent, equal frequencies of rightand left-bending off spring are not consistent with either Model I or II (Figure 2c). In the second genetic marker study, which used both nuclear and mitochondrial DNA markers, Lee et al. [7] report widespread inter breeding between right-and left-bending morphs, consistent with random mating. However, these results are not consistent with the predictions of Model II [6], or with published reports [8], that right-bending males should or do prefer to mate with left-bending females and vice versa (disassortative mating) because RR homozy gotes are lethal.
Off spring frequencies are also inconsistent with two other possible models (Figure 2). Cichlid mouth asymmetry might be inherited like visceral asymmetry in mice [9] (that is, like left-right asymmetry of internal organs such as the heart), where a dominant allele causes mouth bending towards one side but a recessive allele yields random phenotypes (equal numbers of right and left) (Figure 2, Model III). Alternatively, the direction of asym metry might be purely random and not inherited at all, as observed in 27 of 28 cases of random asymmetries in other organisms (Model IV) [10]. Left-bending off spring from two right-bending parents are either too common for model III or too few for Model IV (9 of 10 crosses in both cases; Figure 2). Similarly, right-bending off spring from two left-bending parents are too few for either Model III or IV ( Figure 2).
Unfortunately, direction of mouth-bending of both parents and off spring was scored by eye in all but one cross (that in Figure 2c), even though mouth-bending angles can be diffi cult to detect in adults [3] and are so small as to be 'ambiguous' in more than one-third of fry measured [2]. So the validity of these off spring frequencies is hard to judge. Clearly, more studies like that of Stewart and Albertson [2] are needed to resolve these unsettling inconsistencies.  except rows (a) and (b), where results are pooled from all crosses in that study). Off spring were obtained as broods defended by brooding parents in the fi eld. Parent and off spring phenotypes were scored visually, except in row (c) where off spring phenotypes were measured and parent phenotypes inferred from putative microsatellite markers for right-and left-bending. Colors indicate statistically signifi cant departures from expectations for Model I (orange box), Model II (yellow box), or both Models I and II (blue box). Parent phenotypes follow the original convention of Hori [1], who referred to right-and left-bending as dextral and sinistral. This convention diff ers from [2], who follow the later convention of referring to right-bending fi sh as 'lefties' because they attack prey from the left side [6]. In regard to Model II, if the RR genotype is lethal as proposed in [6], then all right-bending parents (R) must be RL heterozygotes.

Handed behavior and morphological asymmetry
Despite the questions raised above, these recent studies [2,3] suggest an exciting new dimension to the scaleeating cichlid story. Handed or lateralized behavior may amplify morphological asymmetry via developmental plasticity [11]. Handed behavior seems strong and repeatable in these fi sh [1,3]. Hints that it may infl uence morphological asymmetry in P. microlepis come from several sources. First, mouth asymmetry increases with increasing body size [6], as would be expected if repeated lateralized behavior induced greater mouth asymmetry over time. Second, the frequency distribution of mouth asymmetry is not bimodal in brooded juveniles that have not yet begun to feed on scales, suggesting that scaleeating itself may amplify morphological asymmetry later in life [2]. Th ird, mouth asymmetry of fi sh forced experimentally to attack prey from their non-preferred side tended to become less pronounced over time, whereas it increased in those allowed to attack from their preferred side [3]. Fourth, the stronger the lateralized behavior of individual fi sh, the more pronounced the mouth asymmetry [3]. Finally, cichlids, and many other fi sh, exhibit plastic jaw form that responds to the diet they experience [12]: side of attack may therefore matter just as much as kind of prey eaten.
Several puzzling aspects of the scale-eating cichlid story could therefore be more easily explained if mouth asymmetry is actually induced or amplifi ed by lateralized behavior. However, some intriguing questions remain. Does consistency of lateralized behavior increase with increasing body size, as expected if individuals learn to become more profi cient at attacking one side with increasing age? Does lateralized behavior or mouth asymmetry of fi sh reared in the laboratory on a diet of free-fl oating scales become less pronounced compared to fi sh consistently forced to obtain scales by striking the sides of their prey, as expected if lateralized behaviors are learned and morphological asymmetry is amplifi ed by lateralized behavior? Do off spring from left-bending parents exhibit a greater tendency to attack the right sides of their prey even before mouth asymmetry becomes pronounced, as expected if greater genetic variation exists for the degree of lateralized behavior [13] than for the degree of morphological asymmetry? Do those individuals that attack prey consistently from one side obtain more scales than those that sometimes attack from the right or left, as expected if learned behavior improves feeding performance?
As these examples show, studies of right-left asymmetries off er valuable clues about how development evolves because the questions are clear and tests of develop mental mechanisms are possible in many diff erent organisms [10].