Evidence that differences in behaviour are due to genes

The evidence for a genetic basis to behavioural variation is diverse, and much of it, it has to be said, holds little interest for the evolutionary biologist since it emerges from gross disruptions to normal behaviour or other dysfunctional effects (see e.g. van der Steen 1998). However, other lines of evidence have considerable functional/evolutionary interest, and dysfunctional effects can, as we shall see in Chapter 5, have much to say about the cellular developmental processes that lead to changes in behaviour.

2.3 ⁿ Variation and heredity x ₄₅

Figure 2.5 (a) Heritability of a given trait can be demonstrated as a correlation between the trait in a parent and the same trait in its offspring. A close correlation (left) indicates high heritability, while a poor correlation (right) indicates little or no heritability. (b) Heritability can also be demon-strated as a shift in the mean value of a trait as a result of selective breeding. After Ridley (1995).

AB_C02.qxd 9/17/07 8:03 PM Page 45

2.3.2.1 Single genes and behaviour

While no behaviour is encoded at a single locus in a literal sense, there is a wealth of evidence that changes at single loci can affect behaviour through the cascade of develop-mental processes influenced by their expression. Indeed, we have already seen an example for two behaviours in Rothenbuhler’s study of brood care in honey bees, though the role of the loci in question in controlling the expression of the behaviours in Rothenbuhler’s bees remains unclear. Until relatively recently, studies of single gene effects have relied on mutations arising at particular loci and then looking for differences in behaviour between mutant and non-mutant (wild-type) individuals. Useful mutations can arise spontaneously, or, more usually, artificially by exposing organisms to chemical or other mutagens. A useful property of many single gene mutations affecting behaviour is that they also have other phenotypic effects (i.e. they show pleiotropy) so that carriers are easy to identify even when they are not performing the behaviour in question. However, as we shall see in Chapter 5, such pleiotropic effects can also make the interpretation of behavioural differences quite complicated.

A classic example of a single gene mutational effect comes from Bastock’s (1956) study of courtship in male fruitflies (Drosophila melanogaster). Bastock investigated effects on mating success of a sex-linked (carried on one of the so-called X or Y sex chromosomes) recessive mutation, the most obvious effect of which was to change the fly’s body colour from wild-type grey to yellow. Wild-type flies were crossed with the yellow line for seven generations to make sure flies were genetically similar except at the yellow locus. When males from wild-type and yellow lines were allowed to court females, Bastock found that males carrying the yellow allele were much slower to mate and sometimes failed to mate at all. The reason, it turned out, was that yellow males were deficient in crucial elements of the courtship sequence. Courtship in D. melanogaster involves a series of displays and responses by the male, three of which are illustrated in Fig. 2.6. Failure to perform adequately at any stage could compromise a male’s chances of mating. When Bastock analysed the courtship sequence in her flies, she found that the ‘licking’ (contact between the male’s proboscis and the female’s genitalia) and

‘vibration’ (wing movement) phases were longer or more frequent in wild-type males (Fig. 2.6b). Unfortunately for yellow males, it seems that both ‘licking’ and ‘vibration’

serve to stimulate the female and are a necessary prelude to mounting, so courtship tends to abort if they are not performed properly.

As well as demonstrating a behavioural effect of the yellow mutation, Bastock’s study also showed that the effects of the yellow locus depend on the genetic background.

Table 2.1 compares the percentage mating success of wild-type × yellow crosses in stocks where wild-type and yellow lines had not been crossed at all and stocks that had been crossed for seven generations (thus producing similar genetic backgrounds).

In the uncrossed stock there was a significant effect of female genotype (62% vs 87%

and 34% vs 78% in the reciprocal sex/line pairings), whereas no such effect occurred in the crossed stock. This illustrates an important point: the effects of particular alleles on individual reproductive success (and therefore their own fitness [see 2.4.3]) are not absolute but depend on the genetic environment in which they are being expressed. We shall come back to this when we consider the process of adaptation.

Various other single gene mutations in Drosophila have been shown to affect behaviour. Coitus interruptus (in which the male dismounts from the female after only half the normal copulation time) and stuck (where the opposite happens – the male fails to dismount even after the normal copulation time) are two notable ones affecting

Figure 2.6 The temporal patterning of licking (1), vibration (2) and orientation (3) in the courtship sequence of male D. melanogaster of wild-type and yellow stocks. Note the shorter bouts of licking and vibration in yellow males (see text). After Bastock (1967).

Table 2.1 The percentage mating success of wild-type and yellow mutant male Drosophila melanogaster in within and between line crosses and before or after lines had been crossed to generate similar genetic backgrounds. See text. From Bastock (1956).

Before crossing wild stock After crossing wild stock

with yellow stock for with yellow stock for

Matings seven generations seven generations

Wild male ×× wild female 62 75

Yellow male ×× wild female 34 47

Wild male ×× yellow female 87 81

Yellow male ×× yellow female 78 59

AB_C02.qxd 9/17/07 8:03 PM Page 47

sexual behaviour. Many others, such as hyperkinetic, easily-shocked, non-phototactic and dunce, affect movement, responses to environmental stimuli, learning or other aspects of behaviour.

Of course, single gene mutation effects are not limited to Drosophila, but in the main they have been studied in laboratory stalwarts such as fruitflies, laboratory rodents and, more recently, nematodes (e.g. Thomas 1990). One reason is the simple practical one of generating and screening sufficient mutations to identify useful genotypes. Even in laboratory mice, attainable mutation rates, generation times and the colony sizes required can be an enormous disincentive for this kind of study (Takahashi et al. 1994).

Nevertheless, many single gene mutations affecting behaviour in mice have been identified and provide interesting insights into the control of behaviour patterns.

Transgenic techniques

The advent of genetic manipulation techniques has provided opportunities for probing single gene effects more directly. Identified genes can be inserted into (so-called ‘knock-in’ procedures) or removed (‘knock-out’ procedures) from the organism’s cells, a process called transgenesis, or they can be activated and deactivated in situ, to see what effect they have. A nice example comes from the control of circadian activity rhythms in Drosophila. Drosophila melanogaster typically has a 24-hour activity cycle. However, some individuals have shorter (around 19 hours) or longer (around 28 hours) cycles, or do not cycle at all but instead show a random pattern of activity (Fig. 2.7). Breeding experiments have established that these differences arise from alleles of the same gene, known as the period gene (Konopka & Benzer 1971; Baylies et al. 1987). Flies with the wild-type allele, per⁺, show the 24-hour cycle, while those with per^Sor per^Lalleles show the shorter and longer cycles respectively. Flies showing arrhythmic activity patterns carry another allele, per^o. Each of these mutant alleles turns out to differ from the wild-type form by a single pair of nucleotides in the 3,500 pairs that make up the gene (Yu et al.

Figure 2.7 The temporal pattern of activity in Drosophila melanogaster with different mutations of the period gene (see text). After Konopka & Benzer (1971) and Baylies et al. (1987).

1987). By transplanting the fragment of the wild-type fly’s chromosome that contains the per⁺allele into a virus-like vector known as a plasmid, it is possible to transfer the wild-type allele into the cells of a developing mutant fly. When Zehring et al. (1984) did this with mutant per^o(arrhythmic) flies, they found that the adults subsequently exhibited a normal 24-hour activity pattern (Fig. 2.8). The arrhythmic mutants had therefore been genetically transformed into wild-type flies.

The period gene story also illustrates the pleiotropic nature of many mutations affect-ing behaviour. In Drosophila the gene affects not only activity cycles but also the timaffect-ing of eclosion (hatching from the pupa) and male courtship ‘song’ (pulses of sound pro-duced during the wing vibration phase of courtship; see Fig. 2.6a). Variation in song can be detected as differences in the intervals between pulses of sound, usually measured as the inter-pulse interval (IPI), the gap between successive bursts of sound, and the IPI period, the cycle of increasing and decreasing IPIs as the song progresses. Drosophila melanogaster, for example, has IPIs of around 30 milliseconds and an IPI period of about 60 seconds, while D. simulans has IPIs in the region of 50 milliseconds and an IPI period of 35 seconds. Both inter- and intraspecific differences in song pattern appear to be under the control of the period gene. Thus male D. melanogaster with the wild-type per⁺genotype sing normal D. melanogaster song (similarly in D. simulans), while those with the mutant per^o sing arrhythmic song, where the normal pattern of ISIs and ISI periods is broken. Gene transfer experiments have shown that, as with activity patterns, song can be altered predictably by manipulating the per genotype (Wheeler et al. 1991).

Figure 2.9 shows a reciprocal transformation of per^oD. melanogaster and D. simulans males by transferring the per⁺allele of their own or the other species. Thus per^o male

2.3 ⁿ Variation and heredity x ₄₉

Figure 2.8 Arrhythmic per^omutant fruit fly embryos can be transformed into wild-type flies with a normal activity pattern by microinjecting plasmids containing the wild-type per⁺⁺gene. After Konopka & Benzer (1971) and Alcock (1998).

AB_C02.qxd 9/17/07 8:03 PM Page 49

D. melanogaster could be induced to sing normal conspecific song (Fig. 2.9a), or the normal song of male D. simulans (Fig. 2.9b), depending which allele was transferred.

The same was true for D. simulans (Fig. 2.9c,d).

‘Knock-in’ and ‘knock-out’ procedures are also being used extensively in mice to probe the genetic underpinnings of behaviour, particularly sexual behaviour, aggression and learning (e.g. Rousse et al. 1997; Gimenez-Llort et al. 2002; Scordalakes et al.

2002), though questions have been raised as to whether behavioural differences can always safely be attributed to the engineered mutations concerned (e.g. Cook et al.

2002; Rodgers et al. 2002). Despite some fringe doubts, however, genetic manipulations are allowing refined dissection of some of the physiological pathways influencing behaviour, an issue to which we shall return in Chapter 5.

2.3.2.2 Multiple genes

While single gene mutations show that small differences in genotype can have pro-nounced effects on behaviour, many, if not most, behavioural differences are likely to be underpinned by many different loci with complex epistatic relationships between them. The evidence for multiple gene effects is diverse, and much of it long established in the behaviour genetics literature. Some of the key approaches are summarised in Box 2.2, and many other examples can be found in textbooks of the field such as Figure 2.9 Reciprocal transformation of song pattern in male Drosophila melanogaster and D.

simulans. Arrhythmic per^oflies of each species can be transformed to sing the wild-type song of their own or the other species by transferring the appropriate per⁺⁺gene. (a) and (b) show results for D. melanogaster and (c) and (d) for D. simulans. Reprinted with permission after ‘Molecular transfer of species-specific behaviour from Drosophila simulans to Drosophila melanogaster’, Science, Vol. 251, pp. 1082–5 (Wheeler, D. A. et al. 1991). Copyright 1991 American Association for the Advancement of Science.

2.3 ⁿ Variation and heredity x ₅₁