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Evolutionary dynamics and mating behaviour associated with female-limited colour polymorphisms in the damselfly genus Ischnura

Blow, Rachel (2019) BIOP01 20172
Degree Projects in Biology
Abstract
Female-limited colour polymorphisms provide interesting examples of intraspecific genetic diversity but, in most polymorphic taxa, we know little about the macroevolutionary origin and subsequent history of polymorphism during diversification. The damselfly genus Ischnura includes many species with multiple, heritable female colour morphs. The evolutionary origin of polymorphism in this clade is unknown but experimental and field studies in a few polymorphic species of Ischnura indicate that sympatric female morphs, including a male-mimicking morph, are maintained due to sexual conflict over mating, and via negative frequency-dependent selection. Here, I constructed a molecular, time-calibrated phylogeny consisting of 41 out of the 75... (More)
Female-limited colour polymorphisms provide interesting examples of intraspecific genetic diversity but, in most polymorphic taxa, we know little about the macroevolutionary origin and subsequent history of polymorphism during diversification. The damselfly genus Ischnura includes many species with multiple, heritable female colour morphs. The evolutionary origin of polymorphism in this clade is unknown but experimental and field studies in a few polymorphic species of Ischnura indicate that sympatric female morphs, including a male-mimicking morph, are maintained due to sexual conflict over mating, and via negative frequency-dependent selection. Here, I constructed a molecular, time-calibrated phylogeny consisting of 41 out of the 75 species of Ischnura and mapped extant female colour character states with the aim of investigating and understanding the macroevolutionary origin of colour polymorphisms in this genus. In addition, I carried out behavioural experiments under manipulated female colour morph frequencies to determine how behavioural mechanisms and frequency-dependent intersexual interactions maintain a female-limited colour polymorphism in one species, Ischnura elegans. I found evidence of multiple independent evolutionary origins of female colour polymorphism across the Ischnura clade but the ancestral female colour character state in this genus remains uncertain. My phylogenetic analysis indicates weak support for a colour-monomorphic ancestral state, in which females are phenotypically dissimilar from males. This suggests that these polymorphisms could have evolved through the invasion of a novel male-mimicking female morph due to selection pressure from sexual conflict. In addition, I found that female-polymorphisms have arisen repeatedly and that polymorphic lineages have speciated while maintaining these polymorphisms across multiple speciation events and across several millions of years. Finally, I found some significant effects of female morph frequencies and pre-mating male harassment that differed between the three female morphs of I. elegans. Specifically, the amount of male premating harassment towards male-mimicking females increased with this female morph’s own frequency, while the amount of male premating harassment towards the non-male-mimicking females did not vary with their respective frequencies and was consistently high. These results suggest males may have a preference for females that are phenotypically dissimilar from themselves, and male-mimicking females have the selective advantage of reduced male mating harassment but only when at low frequency. Interestingly, the number and degree of male-male interactions (reproductive interference) was also frequency-dependent and increased with the frequency of male-mimicking females. Frequency-dependent male premating harassment towards male-mimicking females is therefore partly responsible for the balancing selection in I. elegans and may even begin to explain how colour polymorphism originated in the Ischnura clade. (Less)
Popular Abstract
On the Origin (and maintenance) of Morphs…

Life on earth is exceptionally diverse but there is still much we do not understand about how genetic diversity is produced and maintained. Species with colour polymorphisms are ideal for studying such evolutionary processes as colour variation within these species often has a simple genetic control and genetically distinct individuals are easy to identify. Due to excessive male mating harassment on females in many species of the pond damselfly genus Ischnura, pressure from sexual conflict has selected for polymorphic females, as males are unable to adapt to mate with any one female morph in particular. In each of these polymorphic species, there is a female phenotype similar to the male... (More)
On the Origin (and maintenance) of Morphs…

Life on earth is exceptionally diverse but there is still much we do not understand about how genetic diversity is produced and maintained. Species with colour polymorphisms are ideal for studying such evolutionary processes as colour variation within these species often has a simple genetic control and genetically distinct individuals are easy to identify. Due to excessive male mating harassment on females in many species of the pond damselfly genus Ischnura, pressure from sexual conflict has selected for polymorphic females, as males are unable to adapt to mate with any one female morph in particular. In each of these polymorphic species, there is a female phenotype similar to the male phenotype and thought to be a male mimic. However, there are also many female monomorphic species in this genus, presumably due to reduced sexual conflict pressure. In this project, I looked into the origination of female-limited colour polymorphisms across the pond damselfly genus Ischnura. I constructed a phylogenetic tree containing 41 out of 75 species in this genus and mapped variation in current and ancestral female colour morph states to determine which mechanisms may have been responsible for the evolution of polymorphism in this genus. I discovered that the most likely female state of the most recent common ancestor of all Ischnura was monomorphic with sexually dimorphic females. This suggests that female colour polymorphism in this genus may have originated several times through the divergence of a male mimicking female morph. In this project, I also investigated the maintenance of female-limited colour polymorphic in the well-studied, trimorphic species Ischnura elegans. Female-limited colour polymorphism in this species is thought to be maintained through negative frequency-dependent selection because males that can switch mating preferences will have higher fitness as they always attempt to mate with the most common female morph whose fitness is consequently lowered. Studies of natural populations suggest there is a positive correlation between female frequency and male mating harassment, however this has never before been studied in a laboratory setting. I therefore carried out laboratory-controlled experiments under manipulated female morph frequencies to determine how this affected the number of male mating interactions. I found that both the number of male-female and male-male interactions increased as the number of male mimics increased in my small populations with manipulated female morph frequencies. This suggests male mimicry, as well as negative frequency-dependent selection, may be important for the maintenance of female-limited polymorphism in this species. In addition, increasing male-male interactions with increasing male mimic frequency may have implications for the indirect effects of male mimicry on population dynamics.


Master’s Degree Project in Biology: Animal Ecology, 60 credits
Department of Biology, Lund University

Supervisor: Erik Svensson
Co-supervisor: Beatriz Willink
Evolutionary Ecology Unit (Less)
Please use this url to cite or link to this publication:
author
Blow, Rachel
supervisor
organization
course
BIOP01 20172
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
8975118
date added to LUP
2019-05-07 10:04:39
date last changed
2019-05-07 10:04:39
@misc{8975118,
  abstract     = {Female-limited colour polymorphisms provide interesting examples of intraspecific genetic diversity but, in most polymorphic taxa, we know little about the macroevolutionary origin and subsequent history of polymorphism during diversification. The damselfly genus Ischnura includes many species with multiple, heritable female colour morphs. The evolutionary origin of polymorphism in this clade is unknown but experimental and field studies in a few polymorphic species of Ischnura indicate that sympatric female morphs, including a male-mimicking morph, are maintained due to sexual conflict over mating, and via negative frequency-dependent selection. Here, I constructed a molecular, time-calibrated phylogeny consisting of 41 out of the 75 species of Ischnura and mapped extant female colour character states with the aim of investigating and understanding the macroevolutionary origin of colour polymorphisms in this genus. In addition, I carried out behavioural experiments under manipulated female colour morph frequencies to determine how behavioural mechanisms and frequency-dependent intersexual interactions maintain a female-limited colour polymorphism in one species, Ischnura elegans. I found evidence of multiple independent evolutionary origins of female colour polymorphism across the Ischnura clade but the ancestral female colour character state in this genus remains uncertain. My phylogenetic analysis indicates weak support for a colour-monomorphic ancestral state, in which females are phenotypically dissimilar from males. This suggests that these polymorphisms could have evolved through the invasion of a novel male-mimicking female morph due to selection pressure from sexual conflict. In addition, I found that female-polymorphisms have arisen repeatedly and that polymorphic lineages have speciated while maintaining these polymorphisms across multiple speciation events and across several millions of years. Finally, I found some significant effects of female morph frequencies and pre-mating male harassment that differed between the three female morphs of I. elegans. Specifically, the amount of male premating harassment towards male-mimicking females increased with this female morph’s own frequency, while the amount of male premating harassment towards the non-male-mimicking females did not vary with their respective frequencies and was consistently high. These results suggest males may have a preference for females that are phenotypically dissimilar from themselves, and male-mimicking females have the selective advantage of reduced male mating harassment but only when at low frequency. Interestingly, the number and degree of male-male interactions (reproductive interference) was also frequency-dependent and increased with the frequency of male-mimicking females. Frequency-dependent male premating harassment towards male-mimicking females is therefore partly responsible for the balancing selection in I. elegans and may even begin to explain how colour polymorphism originated in the Ischnura clade.},
  author       = {Blow, Rachel},
  language     = {eng},
  note         = {Student Paper},
  title        = {Evolutionary dynamics and mating behaviour associated with female-limited colour polymorphisms in the damselfly genus Ischnura},
  year         = {2019},
}