Population Structure and Morph-Specific Fitness Differences in Tristylous Lythrum salicaria

Abstract

In tristylous plant populations, style-morph frequencies are governed by an interaction between frequencydependent selection due to disassortative mating and stochastic processes. Provided that there are no inherent fitness differences among morphs, frequency-dependent selection should result in equal morph frequencies at equilibrium. Stochastic models indicate that the short-styled morph has the highest and the long-styled morph the lowest probability of being lost from local populations as a result of random processes. We surveyed the morph composition of 82 populations of the tristylous, self-incompatible herb Lythrum salicaria in two archipelagos, one in central and one in northern Sweden, located close to the range-margin of the species. To examine whether deviations from even morph frequencies can be explained by among-rnorph differences in reproductive success, we quantified flower and seed production in six and three populations in the northern and southern archipelago, respectively, and we recorded segregation ratios in offspring produced in six trimorphic populations in the northern area. Seed germination and offspring growth were studied in the greenhouse.Ninety percent of the populations in the southern archipelago (N = 31) and 69% of the populations in the northern archipelago (N = 35) were trimorphic; the remaining populations were dimorphic (only populations consisting of at least three flowering plants considered). Dimorphic populations were smaller than trimorphic populations, as predicted by stochastic models. There was a striking difference in the morph composition of L. salicaria populations between the two archipelagos. In the southern archipelago, there was a slight excess of the long-styled morph and a corresponding deficiency of the short-styled morpho In contrast, the northern populations were characterized by a marked deficiency of the mid-styled morph: the average frequency of the mid-styled morph in trimorphic populations was 0.21, and nine of eleven dimorphic populations lacked the mid-styled morpho In both archipelagos, the long-styled morph (the most common morph) produced about 20% fewer seeds per fruit than the other morphs. The long-styled morph also tended to produce fewer seeds per plant. A hand-pollination experiment performed in two of the northern populations indicated that seed production per flower was pollen-limited in the long-styled morph but not in the other two morphs. Seed germination and offspring size after 24 weeks of growth did not differ among morphs. The mid-styled morph tended to have a higher representation in the offspring than in the parental generation in all six trimorphic populations studied further indicating that the deficiency of the mid-styled morph in the northern archipelago does not represent an equilibrium. Taken together, the results do not support the hypothesis that morph-specific differences in reproductive success can account for deviations from even morph frequencies in L. salicaria. It is suggested that among-morph differences in other com...

Phenotypic plasticity of introduced versus native purple loosestrife: univariate and multivariate reaction norm approaches

“…From each population, 20 plants were randomly chosen that were 3 m apart to encourage genetic distinctiveness. Their seeds were considered to be half-sibs of the mother plant, given that purple loosestrife is an obligately outcrossing species (Å gren and Ericson 1996).…”

Stigma receptivity and effects of prior self‐pollination on seed set in tristylous Lythrum salicaria (Lythraceae)

“…Long-styled plants tend to produce fewer seeds and to be more strongly pollenlimited than mid-, and short-styled plants in several Swedish populations Å gren and Ericson, 1996;Waites and Å gren, 2004), whereas short-styled plants produce fewer seeds and are more strongly pollen limited in two North American populations (O'Neil, 1992). Long-styled plants tend to produce fewer seeds and to be more strongly pollenlimited than mid-, and short-styled plants in several Swedish populations Å gren and Ericson, 1996;Waites and Å gren, 2004), whereas short-styled plants produce fewer seeds and are more strongly pollen limited in two North American populations (O'Neil, 1992).…”

“…In Swedish populations of the tristylous, self-incompatible herb, Lythrum salicaria, the long-styled morph tends to produce fewer seeds per flower than the mid-and short-styled morphs because of differences in pollen limitation Å gren and Ericson, 1996;Waites and Å gren, 2004). Moreover, stigmas of the long-styled morph receive considerable amounts of incompatible pollen (Waites and Å gren, 2004).…”

Frequency-dependent sexual selection with respect to progeny survival is consistent with predictions from rock-paper-scissors dynamics in the European common lizard

“…It has been suggested that such fitness intransitivity is the result of sexual selection and that it is the consequence of context-dependent female mate choice (Alonzo and Sinervo, 2001;Sinervo et al, 2007), but few evidence for this assumption exists. RPS games have been described in lizards (Sinervo and Lively, 1996;Sinervo et al, 2007), insects (Zhang et al, 2013), and bacteria (E. coli, Kerr et al, 2002), and it has been suggested by Sinervo and Calsbeek (2006), that they also exist in isopods (Shuster and Wade, 1991), other insects (damselflies, Svensson et al, 2005), and plants (Ågren and Ericson, 1996). In all these species (except in E. coli where direct competition may be the driving force; Kerr et al, 2002), sexual selection exerted by females might be at the origin of fitness intransitivity.…”

“…Thus, sexual selection may again explain the existence of fitness intransitivity, among genetic polymorphisms (Zhang et al, 2013). In the tristylous, self-incompatible herb Lythrum salicaria, three floral morphs (long-styled, mid-styled, short-styled) promote disassortative mating and they exhibit fitness intransitivity, the latter being governed by an interaction between negative FD selection among morphs and stochastic processes (Ågren and Ericson, 1996;Sinervo and Calsbeek, 2006). This suggests that sexual selection on morphs (genetic polymorphisms) is a crucial driver of most of the described RPS games, but thin evidence exists for its existence and importance, and for the critical role of female preference, which might accentuate the outcome of male-male competition (Alonzo and Sinervo, 2001).…”