doctoral dissertation
Katarina Belcijan (Author), Ines Mandić-Mulec (Mentor)

Abstract

Bacillus subtilis is a widespread environmental bacterium capable of kin discrimination during swarming, whith swarms of closely related kin strains merging, while non-kin swarms form strong boundaries. B. subtilis is also able to take up extracellular DNA through competence for transformation. Stress response has been shown to increase the transformation frequency of B. subtilis cells in liquid cultures. Observing activation of stress responses during antagonistic non-kin interactions on swarming agar, we hypothesized that such interactions might promote DNA exchange between non-kin B. subtilis swarms. Our work demonstrates that kinship-dependent interactions among B. subtilis isolates cause envelope stress in the attacked strain, which upregulates the competence in the attacker and facilitates DNA exchange between non-kin swarms. We further examined whether non-kin interactions influence the evolution of divergent kin discrimination (KD) phenotypes. Although no conclusive evidence was found in relation to kinship dependent influence, divergent KD phenotypes emerged and spread more frequently in non- competent populations compared to competence-proficient populations, which denotes the importance of competence in DNA repair and the maintenance of phenotypic traits. The third objective of the thesis was to test whether non-kin encounters limit the spread of exploiters in adjacent populations during surfactin-dependant cooperative swarming over surfaces. Results showed that by mixing surfactin-producing cooperators and surfactin exploiters at 1:1 initial ratio, the cooperative swarming quickly collapses. However, when experimental populations were repeatedly exposed to non-kin or kin swarms, the former were less likely to carry non-swarming clones. This suggests that non-kin encounters constrain the spread of surfactin exploiters and that non-kin attack during swarms’ encounter may influence the stability of cooperation. Overall, our results highlight the importance of social interactions in bacterial evolution and suggest that horizontal gene transfer may play a dual role: promoting adaptation to novel environments while also supporting DNA repair to maintain phenotypic stability. This interplay between genomic plasticity and stability appears to be of central importance to bacterial survival and evolutionary dynamics.

Keywords

Bacillus subtilis;interactions;kin discrimination;experimental evolution;competence for transformation;antagonism;cooperation;DNA exchange;swarming;surfactin;

Data

Language: English
Year of publishing:
Typology: 2.08 - Doctoral Dissertation
Organization: UL BF - Biotechnical Faculty
Publisher: [K. Belcijan Pandur]
UDC: 579.22/.26:579.852.11
COBISS: 232196099 Link will open in a new window
Views: 110
Downloads: 0
Average score: 0 (0 votes)
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Other data

Secondary language: Slovenian
Secondary title: Eksperimentalna evolucija sorodstvenega razlikovanja pri bakteriji Bacillus subtilis
Secondary abstract: Bakterija Bacillus subtilis, ki kolonizira različne ekosisteme, je zmožna sorodstvenega razlikovanja (KD) med kooperativnim rojenjem, kar opazimo kot združevanje rojev visoko sorodnih (»kin«) sevov in nastanek mejnih linij na stiku rojev manj sorodnih (»non-kin«) sevov. Bakterija B. subtilis je zmožna tudi razvoja kompetence za transformacijo in posledično privzema zunajcelične DNA. Predhodne raziskave so pokazale, da okoljski stres poveča frekvenco privzema DNA iz okolja pri bakteriji B. subtilis. Ker antagonizem na mejni liniji rojev dveh manj sorodnih sevov B. subtilis lahko sproži stresni odziv, smo predpostavili, da bi ta lahko vplival na frekvenco izmenjave DNA med roji B. subtilis. Rezultati so potrdili hipotezo in pokazali, da se ob srečanju »non-kin« rojev inducira stresni odziv v napadenem sevu, kar vodi do povečanega izražanja genov za kompetenco pri napadalcu in s tem višje frekvence privzema zunajcelične DNA. Ugotavljali smo tudi ali od sorodnosti odvisne interakcije (izogene, »kin«, »non-kin«) vplivajo na pojavnost klonov, ki tvorijo mejno linijo ob srečanju z rojem starševskega seva. Rezultati niso potrdili razlik v frekvenci spremembe KD fenotipa v odvisnosti od tipa interakcije. Izpostavili pa so povečano divergenco in razširjanje KD fenotipa pri klonih evolviranih populacij mutante, ki ni zmožna privzema DNA. To kaže na vlogo kompetence pri popravilu genoma in posledičnem ohranjanju fenotipskih lastnosti seva. Disertacija tudi naslavlja vpliv sorodstvenega razlikovanja na razširjanje mutant, ki ne proizvajajo surfaktina. Čeprav v mešanici surfaktinske mutante in divjega tipa lahko hitro pride do prevlade mutante in posledičnega kolapsa kooperativnega vedenja, je eksperimentalna evolucija populacij seva B. subtilis pokazala, da so v evolviranih populacijah, izpostavljenih »non-kin« rojem, neroječe mutante manj pogoste, kot v populacijah, ki so bile v ponavljajočem stiku s »kin« rojem. Ta rezultat je v skladu s hipotezo, da napad »non-kin« seva stabilizira kooperativno rojenje. Rezultati disertacije razkrivajo vpliv socialnih interakcij na evolucijo bakterij in skladno s predhodnimi raziskavami kažejo na pomembno vlogo horizontalnega prenosa genov pri popravilu DNA za ohranjanje fenotipske stabilnosti, kot tudi prilagajanju na nove okoljske razmere. To ravnovesje med plastičnostjo in stabilnostjo genomov se zdi ključno za preživetje in evolucijo bakterijskih vrst.
Secondary keywords: Bacillus subtilis;interakcije;sorodstveno razlikovanje;eksperimentalna evolucija;kompetenca za transformacijo;antagonizem;kooperacija;izmenjava DNA;rojenje;surfaktin;
Type (COBISS): Doctoral dissertation
Study programme: 0
Embargo end date (OpenAIRE): 2026-04-08
Thesis comment: Univ. v Ljubljani, Biotehniška fak.
Pages: 1 spletni vir (1 datoteka PDF (XIII, 106 f., [13] f. pril.)
ID: 26197438