By Ana Nicolaci da Costa LONDON (Reuters) - Investors this week will be watching the results of elections that could deal a blow to political parties that are key to reform efforts in the European Union and could also fan instability in Ukraine. The bonds of some struggling euro zone governments sold off last week as investors worried about expected gains for anti-EU parties in European Parliament votes in Greece and Italy. In Italy, a poor result for Prime Minister Matteo Renzi's party could undermine his drive for swift reforms, which he promised when he took power in a party coup earlier this year. The rise of anti-EU parties in northern Europe could make it harder for the European Union to deal quickly with any future resurgence of the euro zone crisis, analysts said.