Expanding branches of a family tree
A couple notes:


I would use classes instead of ids for the nested links. I don't know if it's faster to search the DOM for all elements with a specific class, but it looks cleaner and doesn't require you to create unique ids for each element:

$('a.nested-trigger').click(...);

I would also use classes for the icons, as it's less expensive than replacing DOM nodes:

HTML:

&lt;span class="oc collpased"&gt;&lt;/span&gt;


CSS:

.oc {
  position: relative;
  right: 10px;
  display: inline-block;
  width: 16px;
  height: 16px;
}
.collapsed {
  background: url(icons/bullet_add.png);
}
.expanded {
  background: url(icons/bullet_take.png);
}


JS:

$(this).next("span").removeClass("collapsed").addClass("expanded");

This is related more to the UX of your demo but I would make the icons themselves clickable as well, because they're big enough to look like buttons, and they overlap part of the node.