AJAX :: How To Update Listbox Based On Dropdown Value / Unable To Use Cascadingdropdown

i have a religion dropdown1 , on this selectedindexchange i have to populate castes into the listbox(not dropdownlist).

i am unable to use cascadingdropdown as it is tightly binded with dropdowns.

please guide me how do i update listbox items based on the dropdown selected value using ajax.

i tried with pagemethods getting the dictionary object using JS function and thought of binding it.

but i am unable to bind the list.

//html code
Religion
<asp:DropDownList ID="ddlReligion_RS" runat="server" TabIndex="6" onchange="GetCastes_RS(this.value);">
</asp:DropDownList>
<br />
Caste
<asp:ListBox SelectionMode="Multiple" ID="lstBxCst_RS" runat="server">
</asp:ListBox>
//js function
function GetCastes_RS(relgnID)
{
abc.BLL.Services.AjaxService.GetCastes(relgnID,
function (resp)
{
var ddlCaste= document.getElementById("<%=lstBxCaste_RS.ClientID%>");
ddlCaste.length=0;
var castesList = resp;
alert (resp);
if(castesList.keys.length >0)
{
for(var i=0 ;i< castesList.keys.length;i++)
{
op=castesList.keys[i];
ddlCaste.options[ddlCaste.length]=new Option(castesList.values[i],op);
}
}
},
function(val)
{
alert(val.get_message());
}
);
}

i could call the webmethod, but i am getting this error in javascript.

Type 'System.Collections.Generic.Dictionary`2[[System.Int32, mscorlib, Version=2.0.0.0, Culture=neutral,
PublicKeyToken=b77a5c561934e089],[System.String, mscorlib, Version=2.0.0.0, Culture=neutral,
PublicKeyToken=b77a5c561934e089]]' is not supported for serialization/deserialization of a dictionary, keys must be strings or objects.

when gone thru few links i have seen few examples supporting dictionary object , why not in my case?