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Reproductive barriers in two closely related damselfly species

Giraud, Aude (2016) BIOM01 20161
Degree Projects in Biology
Abstract
Hybridization between species can be avoided by behavioural or physiological mechanisms that act as prezygotic and postzygotic reproductive barriers. In sympatric areas, where species come into contact, the maintenance of species distinctness will depend on the strength of such mechanisms. In cases with recent secondary contacts of closely related species, barriers are often incomplete and the outcome can be either introgressive hybridization (and ultimately merging of the species) or rein-forcement (selection for stronger prezygotic barriers). In this paper I used Ischnura graellsii and I. elegans as an example of such case, and studied prezygotic (two premating and three postmating) reproductive barriers between these species from... (More)
Hybridization between species can be avoided by behavioural or physiological mechanisms that act as prezygotic and postzygotic reproductive barriers. In sympatric areas, where species come into contact, the maintenance of species distinctness will depend on the strength of such mechanisms. In cases with recent secondary contacts of closely related species, barriers are often incomplete and the outcome can be either introgressive hybridization (and ultimately merging of the species) or rein-forcement (selection for stronger prezygotic barriers). In this paper I used Ischnura graellsii and I. elegans as an example of such case, and studied prezygotic (two premating and three postmating) reproductive barriers between these species from different geographic populations. The results show that complete isolation is almost achieved in all crosses by prezygotic barriers, but that the contribu-tion of each barrier to isolation varies depending on the population. More specifically, premating isolation is overall higher than other barriers, and there are asymmetries between species, with bar-riers being stronger in I. graellsii than in I. elegans. There is also a clear sign of reinforcement through the higher isolation in sympatry than in allopatry, but the high degree of introgression in I. elegans can counterbalance the reduction of gene flow through barriers between the two species. Thus, the long-term evolutionary outcome of the hybrid zone may still be uncertain. A further per-spective will be to assess the possibility of within-species speciation in I. elegans. (Less)
Popular Abstract
Why can't these damselfly species be lovers?


Matings with a partner from the wrong species can be avoided by mechanisms involving the behaviour or the physiology of an individual. These mechanisms litterally act as barriers to reproduction, and happen before or after the formation of the embryo. It means that, through these mechanisms, two species will become reproductively isolated from each other. However, in areas where two different species can meet, these barriers are sometimes not strong enough. This is particularly important when the two species are closely related within their family, like would be two sisters. When there are many matings between the two species, the result can be the formation of hybrids and the transfer of... (More)
Why can't these damselfly species be lovers?


Matings with a partner from the wrong species can be avoided by mechanisms involving the behaviour or the physiology of an individual. These mechanisms litterally act as barriers to reproduction, and happen before or after the formation of the embryo. It means that, through these mechanisms, two species will become reproductively isolated from each other. However, in areas where two different species can meet, these barriers are sometimes not strong enough. This is particularly important when the two species are closely related within their family, like would be two sisters. When there are many matings between the two species, the result can be the formation of hybrids and the transfer of genes from one species to another (introgression). It can also be that the barriers evolve stronger (reinforcement). These two processes are important in the understanding of species distinctness and formation.

Here I present five barriers in females, happening before the embryo formation, between two damselfly sister species, Ischnura elegans and I. graellsii. I. elegans is expanding from western Europe to Spain, where I. graellsii lives. Therefore it is an opportunity to understand how the barriers are evolving between two species, and which ones prevent most the matings. The individuals come from different geographic populations (in which the species are in a contact area, in the boundaries of this area or geographically isolated), hence it is possible to understand the differences in barrier strength between the species and also the populations.

Mating experiments, where males and females are put together, show that the barriers are strong enough to prevent matings (or at least the formation of the embryo), but each type of barrier have a different contribution depending on the species and the population. Those that occur before the act of copulation (here because of morphological dissimilarities) are the most prevalent. Moreover, the barriers are stronger in I. graellsii than in I. elegans. Additionally, I. elegans females from the contact area show stronger barriers than females from the isolated area (which typically represent reinforcement). On the other hand, many genes from I. graellsii are being transfered to I. elegans by hybridization, but not in the opposite direction. It means that the barriers could become weaker, allowing for matings between the species to be fertile. If the genes give an advantage to I. elegans in being better adapted to the environmental conditions in Spain, this species can expand even more and outnumber I. graellsii.

In summary, these two damselfly species cannot be lovers because of barriers in reproduction. The outcome of the contact area is still uncertain. Either the two species will definitively be isolated, which means that no matings will be possible at all, or it can lead to the extinction of I. graellsii as the expansion of I. elegans progresses. An interesting further perspective regarding these barriers is the difference in strength between I. elegans populations. As barriers are stronger in the contact area than in the isolated area, it could be that later on these two populations will not be able to mate between each other. In the very long term view, this can result in the formation of two different species.

Supervisors: Bengt Hansson, Rosa Ana Sánchez-Guillén
Master Degree Project in Biology, 30 credits, 2016
Department of Biology, Lund University (Less)
Please use this url to cite or link to this publication:
author
Giraud, Aude
supervisor
organization
course
BIOM01 20161
year
type
H2 - Master's Degree (Two Years)
subject
language
English
id
8891685
date added to LUP
2016-09-14 10:52:29
date last changed
2016-09-14 10:52:29
@misc{8891685,
  abstract     = {Hybridization between species can be avoided by behavioural or physiological mechanisms that act as prezygotic and postzygotic reproductive barriers. In sympatric areas, where species come into contact, the maintenance of species distinctness will depend on the strength of such mechanisms. In cases with recent secondary contacts of closely related species, barriers are often incomplete and the outcome can be either introgressive hybridization (and ultimately merging of the species) or rein-forcement (selection for stronger prezygotic barriers). In this paper I used Ischnura graellsii and I. elegans as an example of such case, and studied prezygotic (two premating and three postmating) reproductive barriers between these species from different geographic populations. The results show that complete isolation is almost achieved in all crosses by prezygotic barriers, but that the contribu-tion of each barrier to isolation varies depending on the population. More specifically, premating isolation is overall higher than other barriers, and there are asymmetries between species, with bar-riers being stronger in I. graellsii than in I. elegans. There is also a clear sign of reinforcement through the higher isolation in sympatry than in allopatry, but the high degree of introgression in I. elegans can counterbalance the reduction of gene flow through barriers between the two species. Thus, the long-term evolutionary outcome of the hybrid zone may still be uncertain. A further per-spective will be to assess the possibility of within-species speciation in I. elegans.},
  author       = {Giraud, Aude},
  language     = {eng},
  note         = {Student Paper},
  title        = {Reproductive barriers in two closely related damselfly species},
  year         = {2016},
}