How to populate second dropdownlist without postback? - asp.net

Question

I have many dropdownlists and textboxes in a single page. I have done validations on textboxes. Each dropdownlist populate another dropdownlist.
The problem is, all existing validation messages are disappear when selectedindexchanged event fired. How to populate second dropdownlist without postback?
<asp:Content ID="Content1" ContentPlaceHolderID="mainContentPlaceholder" runat="server">
<asp:Panel ID="Panel1" runat="server" CssClass="formattingPanel" ScrollBars="Vertical">
<asp:UpdatePanel ID="UpdatePanel1" runat="server" ChildrenAsTriggers="true" UpdateMode="Conditional" >
<ContentTemplate>
<table>
<tr>
<td>Title </td>
<td><asp:DropDownList ID="ddlTitle" runat="server" OnSelectedIndexChanged="ddlTitle_SelectedIndexChanged" AutoPostBack="true" />
<asp:TextBox ID="txtOther" runat="server" Visible="false" Text="" Width="33%"/>
</td>
</tr>
<tr>
<td> Nname </td>
<td> <asp:TextBox ID="txtName" runat="server" /> </td>
</tr>
<tr>
<td> Address1 </td>
<td> <asp:TextBox ID="txtAddress1" runat="server" /> </td>
</tr>
<tr>
<td> Address2 </td>
<td> <asp:TextBox ID="txtAddress2" runat="server"/> </td>
</tr>
<tr>
<td> Postcode </td>
<td> <asp:TextBox ID="txtPostcode" runat="server" /> </td>
</tr>
<tr>
<td> Telephone </td>
<td> <asp:TextBox ID="txtTelephone" runat="server" />
<asp:RegularExpressionValidator ID="regexTelephone" runat="server" ControlToValidate="txtTelephone" ErrorMessage ="* Numbers only & no spaces"
ValidationExpression ="^\d+" ForeColor="Red"> </asp:RegularExpressionValidator>
</td>
</tr>
<tr>
<td> Email </td>
<td> <asp:TextBox ID="txtEmail" runat="server" />
<asp:RegularExpressionValidator ID="regexEmail" runat="server" ValidationExpression="\w+([-+.]\w+)*#\w+([-.]\w+)*\.\w+([-.]\w+)*"
ControlToValidate="txtEmail" ErrorMessage="* Invalid Email" ForeColor="Red" > </asp:RegularExpressionValidator>
</td>
</tr>
<tr>
<td>Title 1</td>
<td>
<asp:UpdatePanel ID="UpdatePanel2" runat="server" ChildrenAsTriggers="true" UpdateMode="Conditional">
<Triggers>
<asp:AsyncPostBackTrigger ControlID="ddlType1" EventName="SelectedIndexChanged" />
</Triggers>
<ContentTemplate>
<asp:DropDownList ID ="ddlType1" runat ="server" OnSelectedIndexChanged="ddlType1_OnSelectedIndexChanged" AutoPostBack="true" />
<asp:DropDownList ID ="ddlSubType1" runat ="server" Visible="false" />
</ContentTemplate>
</asp:UpdatePanel>
</td>
</tr>
<tr>
<td> Title 2 </td>
<td>
<asp:DropDownList ID ="ddlType2" runat ="server" OnSelectedIndexChanged="ddlType2_OnSelectedIndexChanged" AutoPostBack="true"/>
<asp:DropDownList ID ="ddlSubType2" runat ="server" Visible="false"/>
</td>
<tr>
<tr>
<td> Title 3 </td>
<td>
<asp:DropDownList ID ="ddlType3" runat ="server" OnSelectedIndexChanged="ddlType3_OnSelectedIndexChanged" AutoPostBack="true"/>
<asp:DropDownList ID ="ddlSubType3" runat ="server" Visible="false"/>
</td>
<tr>
---- upto 8 dropdownlist sams as above
<tr>
<td> <asp:Button ID="btnSave" runat="server" Text ="SAVE" OnClick="btnSave_Click"/> </td>
</tr>
</table>
</ContentTemplate>
</asp:UpdatePanel>
</asp:Panel>
</asp:Content>
I have done ddl binding using c# code..