AJAX :: Objectdatasource Not Selecting In Updatepanel?

I have an updatepanel with detailsview in it. Detailsview contains two template items, Country and State. In the readonly state, label is used to dispaly the values. In the Edit and Insert mode, dropdownlist is used. Objectdatasource is set as the datasource for dropdownlist. When the selectedindex changed for country dropdownlist, the corresponding states should be fetched. I have set a select parameter in state objectdatasource. I could not set controlparameter in state datasource as in the readonly mode, dropdownlist is not available. So, i used onselecting event of state datasource to set the value(i.e selectedvalue of the coutry). But what i observed is that seletiing event is not firing when a postback happens because of coutry selectedindex change. I found that post back is happening due to the event and country datasource is selecting, but it si not refreshing the state list.