Can someone explain to me why Bosnia has almost 3x i2a over r1a and why bulgaria has equal of both.
I do not know for sure but the reasons can be multiple (genetic drift, bottlenecks, more or less succesful expansions after the 7th century). Besides, we also have to take into account that the original genetic make up of said tribes was not identical (which is also reflected by the frequencies and deep subclades present today). Besides, it is worth noting that Slavs and proto-Bulgarians were distinct tribes, although they arrived in Bulgaria almost at the same time. The initial homeland of the proto-Bulgarians was in the foothills of Pamir and Hindu Kush (Balhara for the Indians/Bactria for the Greeks), which could justify some Y DNA differences. Subsequently, they founded their European realms (i) Old Great Bulgaria between the Caucasus, the Caspian Sea and the Dnieper River, (ii) Volga-Kama Bulgaria and (iii) Danubian Bulgaria (Asparukh).
According to
Primorac 2011, Bosnian males have 50 % of Hg I and 13,7 % of R1a (while in Croatia, Hg I represents 49 % of the fatherlines and Hg R1a 27 %). In summary, the authors write that:
(i) Hg I (mutation P37) is very ancient in Europe (25'000 ybp);
(ii) it expanded from a Western LGM refugium in the Balkans; and
(iii) it likely contributed to the post LGM peopling of Bosnia and Croatia.
I personally think that this reasoning is not convincing, in terms of continuity, if you look at the young age of the deep subclades present in those territories and at their limited diversity.
Regarding R1a, the authors state that this Hg was possibly introduced in Croatia from the Northern part of Eastern Europe through the widespread of the Corded Ware cultures (3200/2300 BC) down through the more recent Slavic expansions. Finally, they conclude that R1a is the second most frequent haplogroup in the mainland and island populations, which implies that at least some of the founding ancestral groups of Croats originated from populations having possibly migrated from southern Russia
2000 ybp. As for the lower R1a frequency in Bosnia, the authors simply conclude that the Bosnian population shows a smaller portion of genes for the Ukranian refugium.
From what I see in
Karachanak's study of 2013, Bulgarians have 20,2 % of I-M423, 18,1 % of E-V13 and 17,5 % of R1a (described in the supporting information as R-M17, 43 % of which being R-M458). The authors write about Bulgarian R1a that this haplogroup could be a signal of various events ranging from early post LGM expansions to more recent Slavic demography.
As I wrote earlier, it all depends from the specific subclades and their age.