Código PHP:
<select name="asesor" id="asesor" align="right" style="width:180px">
<?php
$a="select A.*, U.*
from asesores as A
left join usuario as U on A.id_asesor=U.id_asesor";
$aa=mysql_query($a,$link);
while($on=mysql_fetch_array($aa)){
echo '<option value="'.$on["id_asesor"].'" selected>'.$on["nombres"].'</option>';
}
?>
</select>