TY  - THES
T1  - Speciation and isolating barriers in a parasitoid wasp focusing on the role of reproductive isolation caused by endosymbionts
A1  - Pollmann,Marie
Y1  - 2024/01/10
N2  - Hymenoptera are one of the most speciose animal taxa, presumably caused by a high rate of speciation. The investigation of evolutionary processes is predominantly based on the biological species concept (BSC), which defines species as groups of interbreeding individuals which are reproductively isolated from individuals of other groups. As an alternative approach, species are delimitated by predetermined threshold distances in the so-called barcode segment of the mitochondrial COI gene. Many arthropods are infected with endosymbiotic bacteria, some of which have been shown to manipulate their hosts’ reproduction via mechanisms like cytoplasmic incompatibility (CI). It occurs between infected males and uninfected females and results in the reduction or absence of diploid offspring. Due to their ability to interfere with the reproduction of their hosts, endosymbionts have been suggested to be potential drivers of their hosts’ speciation processes.
Lariophagus distinguendus is a parasitoid wasp which uses larvae of multiple coleopteran species as hosts. In past studies, two distinct species of L. distinguendus have been discovered. They are separated by different barriers including unidirectional CI caused by an unknown bacterium. 
Here, the nature of the CI inducer in crosses between untreated males and tetracycline-treated females of the L. distinguendus strain STU was investigated. Additionally, diversity and speciation processes were investigated within the L. distinguendus species complex by constructing phylogenetic trees with COI and nuclear genes as well as by conducting crossing experiments with different strain combinations.
The bacterium Spiroplasma, termed sDistinguendus (sDis) for its L. distinguendus host, was identified to be the causative agent of this CI. Despite being known for multiple types of interactions with various hosts, Spiroplasma had not been demonstrated to cause CI before. None of the other bacteria identified within STU were associated with CI and none of the known CI inducers and other bacteria manipulating reproduction were found. The potential for maternal transmission was shown by the presence of sDis in the ovaries of STU females. Transferring sDis and CI from infected to uninfected STU females via hemolymph microinjection solidified the connection between CI and sDis. The simplicity of this transfection technique could prove to be valuable for applicable purposes in future. CI strength varying between male offspring of the injected females was not linked to the titer of sDis.
All available L. distinguendus strains were analyzed with the barcode segment of COI as well as five nuclear genes to infer their phylogenetic relationships. Furthermore, five pairs of strains covering a gradient of genetic divergence were selected for crossing experiments investigating their reproductive isolation. The nuclear genes recovered two clusters identical to the previously known species, whereas three distinct clusters were found based on barcoding. These three clusters were revealed as separate species due to very high to complete reproductive isolation in crossing experiments between strains representing each of them. Threshold values of COI distances commonly used for species delimitation relying predominantly on barcodes, i.e. 2% to 2.2%, were shown to be not applicable for L. distinguendus due to the divergence between strains of the same species exceeding these thresholds. This calls the faith in these pre-set thresholds and the barcode-only approach to species delimitation into question and suggests the consideration of additional data along with those obtained by barcoding for species delimitation. Reproductive isolation between the strains was found to span a continuum of divergence from no reproductive isolation and low genetic divergence to complete isolation and high genetic divergence. The first barrier, present in all strain pairs, was sexual isolation, ranging from weak and unidirectional to complete and near complete in both directions. In the untreated cross between BIR females and STU males, sDis-induced CI was present along with sexual isolation. The reduced fertility of hybrid males was the next barrier to appear in the three more distantly related strain pairs along with behavioral sterility of hybrid females in the one strain pair among them where it was measured. These barriers were followed by the behavioral sterility, inviability, and physiological sterility of hybrid males and as last barrier by the reduced fertility of hybrid females. Due to the majority of barriers affecting hybrid males, but not females, speciation in L. distinguendus follows Haldane’s rule, stating that a fitness loss present in the hybrids of only one sex will be affecting the heterogametic sex, i.e. the males in L. distinguendus.
In conclusion, the CI between infected males and uninfected females of L. distinguendus was found to be caused by Spiroplasma, termed sDis, marking the first time for CI to be demonstrated as phenotype of Spiroplasma. It was also one reproductive barrier present in crossings of different L. distinguendus strains, apparently contributing to their divergence. The diversity within the L. distinguendus species complex was shown to be greater than detected before, as seen by the discovery of a third species. The studied pairs of strains covered a wide range of both genetic divergence and strength of reproductive isolation, suggesting L. distinguendus to be currently in the process of ongoing speciation. The commonly used threshold distances in COI were not applicable for species delimitation in L. distinguendus, challenging their unconditional use.
KW  - Artbildung
KW  - Artabgrenzung
KW  - Lariophagus distinguendus
KW  - Spiroplasma
KW  - cytoplasmatische Inkompatibilität 
CY  - Hohenheim
PB  - Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim
AD  - Garbenstr. 15, 70593 Stuttgart
UR  - http://opus.uni-hohenheim.de/volltexte/2024/2261
ER  -