Att förstå hur evolutionen av hjärnan resulterat i den enorma variation i hjärnmorfologi vi ser bland nu levande ryggradsdjur har fascinerat forskare i årtionden. Hjärnans morfologi bestäms både genetiskt och av omgivande miljöbetingelser. Till exempel anpassningar till ekologiska utmaningar har föreslagits vara en viktig drivkraft i hjärnans diversifieringsprocesser. Nervvävnad är mycket energikrävande och evolutionen av hjärnan är därför troligen en mycket komplex process som involverar en delikat balans mellan kostnader och fördelar.
Jag använde mig av guppyn (Poecilia reticulata) som modellorganism för att först undersöka under vilka förhållanden diversitet i hjärnmorfologi genereras. Jag gjorde detta genom att undersöka miljömässiga och kognitiva faktorer, som man sedan tidigare vet kan påverka hjärnplasticitet (Kapitel I). Tidigare studier har visat att ökad miljöberikning leder till en förstoring av vissa delar av hjärnan. Medan ökad miljökomplexitet ledde till plastiska förändringar i hjärnmorfologi fann jag att inga signifikanta skillnader efter framgångsrikt genomförande av två kognitiva test.
Jag undersökte sedan möjligheten för mosaikevolution av hjärnan genom att artificiellt selektera för ökad och minskad telencephalonstorlek (Kapitel II).
Telencephalonstorlek visade sig svara snabbt på divergerande selektionstryck medan övriga hjärnregioner var oförändrade. Telencephalonstorleken hos fisk från den icke-selekterade kontrollbehandlingen skiljde sig inte från den hos vildtypsfiskar, medan både upp- och nedselekterade fiskar hade större respektive mindre telencephalon.
Fisk från de artificiella selektionslinjerna testades sedan i tre olika tester för att undersöka om skillnader i telencephalonstorlek påverkar djärvhet (Kapitel III). Tiden det
tog för individer att komma ut från ett skydd och tendensen att undersöka en tom arena och ett okänt föremål undersöktes. Jag fann inga skillnader i djärvhet mellan de olika selektionslinjerna, men hanar visade sig vara mer djärva och aktiva än honor.
De kognitiva fördelarna med att ha en stor telencephalon undersöktes i ett självinhiberingstest (Kapitel IV). Guppies från de uppselekterade linjerna lärde sig snabbare och gjorde fler korrekta vägval jämfört med de nedselekterade linjerna.
Sammanfattningsvis har jag funnit experimentellt stöd för hypotesen om mosaikevolution av hjärnan genom att visa att olika hjärnregioner kan evolvera snabbt och oberoende av andra regioner under riktad selektion. Framtida test av kostnader, fördelar i andra kognitiva test och detaljerade undersökningar av nervvävnaden kommer att hjälpa oss att vidare reda ut vilka faktorer som ligger bakom evolutionen an hjärnan.
ACKNOWLEDGEMENTS
First and foremost, I would like to thank my supervisor, Niclas Kolm, for giving me this unique opportunity to be part of this once-in-a-lifetime project. Niclas, thanks for being a consistent supportive force, celebrating every little win with my selection experiment and helping me push forward throughout the past 4+ years. I faced many obstacles and setbacks over the years, but you believed in me and inspired me to never give up.
This taught me to be more resilient and to adopt a generally more positive outlook, something I am ultimately grateful for. I would also like to acknowledge my co-supervisor, Björn Rogell, who has no doubt assisted and taught me a lot regarding statistics, and for the constant encouragement.
I owe much gratitude to the rest of the Kolm lab: Mirjam, Annika and Zegni. Mirjam, you were always willing to lend a helping hand no matter how busy you were (which you always were), and I greatly appreciate the countless number of times I approached you for advice. Of course, a huge thank you for your help with the translation of my thesis summary as well. Annika, you were always around the department even before I started, so I knew I could turn to you for guidance in the lab. You have helped me more than you know and I thoroughly enjoyed all our conversations, be it work related or not, and of course attending our first conference together. Zegni, you have been a mentor to me during this past year, giving me valuable feedback on my manuscripts and experiments.
Thanks for all the fascinating discussions we have had, I have definitely learnt a lot from you.
Not forgetting previous members of the lab, Séverine, Alex, Wouter, Alberto and Teddy.
Séverine, you were nothing but welcoming when I first moved to Stockholm, helping make the move much easier. When I first started at the department, I knew I could always turn to you for help and you were always around to assist in any way you could. You have been a huge influence during my PhD journey, and I am thankful for that. Alex, if it weren’t for your constant guidance, I would have been entirely lost with my selection experiment.
You showed me the steps needed for such a huge undertaking, and taught me how to do my first dissection. Wouter, aka R guru, I owe a great debt to you for your willingness to help me with my analyses even after you moved to the faraway land of Canada. I have definitely learnt a lot about R from you. Alberto, thanks for providing advice with my experiments when needed and volunteering your time to help me resolve whatever problems I encountered in the lab. Teddy, you were not only someone I could turn to for advice during the start of my PhD, but you were an awesome office mate with all your jokes and constant discussions about Eurovision. I would also like to thank Vivien Holub, who was tasked with the huge responsibility of looking after thousands of my fish in the lab. I cannot emphasize enough how much easier you made my life but ensuring that my fish were well taken care of.
Like any other PhD student, I started my journey feeling extremely optimistic and excited.
But over the years, it was the encouragement and support from friends and colleagues that saw me through this journey. Erika, you were always bugging me to speak Swedish, correcting (and laughing at) me whenever I got it wrong. But I am grateful that you took the effort to help, and of course for our many outings/dinners/trips. Maddi, you have always been there with words of encouragement and believed in me. You have been a
true friend and continue to be a great help in the lab. Regina, I truly appreciate all the help and advice you have given me. You would always take time out and try your best to help me. Hannah, thanks for listening to me rant, for discussing statistics with me and for always making me laugh with your witty humour. Laura, thank you for being someone I knew I could always turn to. You have been a great friend, teammate, gossip companion and so on, and always had my back. You guys have always been there for me, through all the sweat and tears and to constantly encourage me to power though, and words simply cannot express how grateful I am for everything you have done for me. When I first moved to Stockholm, I was fortunate enough to be able to live together with two other students.
To Despina and Cathy (aka my pumpkins as affectionately labelled), thanks for always being so supportive and believing that I would make it to the end. And of course, to all my innebandy teammates in Sumpan, you guys have kept me sane. I always look forward to trainings after a stressful day at work and you have all helped to keep me positive and going strong.
To everyone in the department, each and every one of you have made working here much more enjoyable and memorable and I am extremely appreciative for everything. To my office mates, Malin, Ciaran, Amy, thanks for all the fun times we had pre-covid, and for making our shared office space one filled with productivity and laughter.
It goes without saying that I am exceptionally grateful for the support I have received from my family despite the distance and time difference. My parents, Catherine and Michael, have always supported me when I wanted to pursue my dream, even if it meant me moving overseas. Thanks for always believing in me and for pushing me to venture outside
of my comfort zone. To my sisters, Sam and Sarah, you two have been my constant support system through it all, and I am super grateful for having you both as my siblings.
And a special shout out to my amazing designer sister for helping me with the cover page of my thesis.