Contrasting Patterns of Heritable Geographic Variation in Shell Morphology and Growth Potential in the Marine Gastropod Bembicium vittatum: Evidence from Field Experiments

Abstract

Similar phenotypes do not always imply similar genotypes. In species distributed over a broad latitudinal range, geographical variation in morphological and life-history traits may reflect very different relations between genotypic and environmental effects on these traits. Patterns of selection among latitudinally separated sites may minimize phenotypic differences in life-history traits but promote phenotypic differences in form. Thus, for example, latitudinal variation in temperature often leads to genetically based metabolic differences that minimize differences in growth rate among populations at different latitudes (countergradient variation). However, variation in habitat experienced by the same populations may promote genetically based differences in shell form (cogradient variation). Few attempts have been made to assess simultaneously such mosaic effects of natural selection on the genetic basis of variation in both morphological and life-history traits among geographically separated populations. I quantified the extent to which widely separated populations of the rocky shore marine gastropod Bembicium vittatum exhibited genetic differences in shell shape, shell pattern, and growth rate. Bembicium vittatum occurs naturally at only three widely separated locations on the Western Australian coast. Individuals were transplanted from all three locations to a latitudinally intermediate site, where they were released in different pairwise combinations and allowed to reproduce. F offspring from crosses between same- or different-source parents were identified using allozyme markers. When grown in a common environment, offspring from same-source parents exhibited similar differences in shell shape and pattern, but dramatic differences in growth rates, compared to native populations. Genetic variation therefore exists for all three traits. Growth rates in the common environment were positively correlated with latitude of the source population, confirming the existence of countergradient variation for growth associated with metabolic compensation. In addition, for both shell shape and growth rate, hybrids exhibited phenotypes roughly midway between the same-source parents, suggesting that genetic differences have a large additive component. In contrast, when one parent had pigmented spots, the offspring also had spots, suggesting a strong dominance component to the genetic basis of shell pattern. Genetic differences therefore yield different morphological phenotypes but similar life-history phenotypes, among latitudinally distant populations, and confirm a pattern of mosaic evolution in B. vittatum.

Experimental Studies of Adaptation in Clarkia Xantiana. I. Sources of Trait Variation Across a Subspecies Border

“…Thus flowering time at the east site would have been even later if node of first flower had not declined in concert. This exemplifies the idea that if one trait's unavoidable response to the environment is indeed maladaptive, harmful effects may be ameliorated by compensatory responses in other traits (Levins 1968;Berven and Gill 1983;Conover and Schultz 1995;Parsons 1997;Preston 1999).…”

“…The commonness of CnGV in fitness-related traits appears to be poorly known in general, though it has been found, especially in animal systems, that CnGV in growth may commonly accompany (and underlie) invariance or CoGV in mature size and form (Conover and Schultz 1995;Parsons 1997;Trussell 2000).…”

Differential shell strength of Cepaea nemoralis colour morphs—implications for their anti-predator defence

“…However, a study on marine gastropod Bembicum vittatum showed a strong genetic basis of its shell morphology (Parsons 1997) which is an important factor affecting shell strength (DeWitt 2000; Rundle and Brönmark 2001). On the other hand, shell strength could be strongly shaped by the environment, e.g.…”

Adaptation to climate change: contrasting patterns of thermal-reaction-norm evolution in Pacific versus Atlantic silversides

“…The most common form of CnGV involves metabolic compensation and is displayed mostly in physiological traits, for example, in the genetically higher growth capacities of many poikilotherms at high versus low latitudes (e.g. [7] (Reptilia); [8] (Pisces); [9] (Insecta); [10] (Gastropoda); [11] (Amphibia)). In cases of CnGV in growth, genetic growth capacities are adjusted upward or downward over a species' entire viable temperature range, with thermal reaction norms (¼temperature-specific phenotypic trait expressions, figure 1) shifted in parallel to higher or lower levels.…”