一个英语语法问题

Russian gay rights activists were joined by campaigners from America and Europe in what has become an annual event.

这里的what 引导的是什么从句或者解释一下句子的结构?

答：这是一个被动语态的句子。主语是：Russian gay rights activists

谓语：were joined

状语：by campaigners

定语：from America and Eruope

状语：in +名词性从句，表示在。。。事件中。

名词性从句分析：主系表结构

主语：what 相当于名词+that- 定语从句。

系动词 has become

表语：an annual event

译：用主动翻译，

来自美国和欧洲的活动家们在现以成为每年一次的事件中加入了俄罗斯的“同志”权益激进派的行列。