Attempting genetic inference from directional asymmetry during convergent hindlimb reduction in squamates

Loss and reduction of paired appendages is common in vertebrate evolution.
How often does such convergent evolution depend on similar developmental
and genetic pathways? For example, many populations of the Threespine
Stickleback and Ninespine Stickleback (Gasterosteidae) have independently
evolved pelvic reduction, usually based on independent mutations that
caused reduced Pitx1 expression. Reduced Pitx1 expression has also been
implicated in pelvic reduction in manatees. Thus, hind limb reduction
stemming from reduced Pitx1 expression has arisen independently in groups
that diverged tens to hundreds of millions of years ago, suggesting a
potential for repeated use of Pitx1 across vertebrates. Notably, hindlimb
reduction based on reduction of Pitx1 expression produces left-larger
directional asymmetry in the vestiges. We used this phenotypic signature
as a genetic proxy, testing for hindlimb directional asymmetry in six
genera of squamate reptiles that independently evolved hindlimb reduction
and for which genetic and developmental tools are not yet developed:
Agamodon anguliceps, Bachia intermedia, Chalcides sepsoides, Indotyphlops
braminus, Ophisaurus attenuatuas and O. ventralis, and Teius teyou.
Significant asymmetry occurred in one taxon, Chalcides sepsoides, whose
left-side pelvis and femur vestiges were 18% and 64% larger than
right-side vestiges, respectively, suggesting modification of Pitx1
expression in that species. However, there was either right-larger
asymmetry or no directional asymmetry in the other five taxa, suggesting
multiple developmental genetic pathways to hindlimb reduction in squamates
and vertebrates more generally.

脊椎动物进化中，配对附肢的丢失与减少现象十分普遍。
这种趋同进化（convergent evolution）在多大程度上依赖相似的发育和遗传通路（developmental and genetic pathways）？例如，三刺鱼（Threespine
Stickleback）和九刺鱼（Ninespine Stickleback，属刺鱼科Gasterosteidae）的多个种群均独立
进化出骨盆减少（pelvic reduction）特征，通常源于导致Pitx1表达降低的独立突变。Pitx1表达降低也被
证实与海牛的骨盆减少有关。因此，由Pitx1表达降低引发的后肢减少（hind limb reduction），在分化了数千万至数亿年的类群中
独立出现，这表明Pitx1在脊椎动物中具有重复利用的潜力。值得注意的是，基于Pitx1表达降低的后肢
减少会在残迹结构（vestiges）中产生左侧更大的方向不对称性（directional asymmetry）。我们将这一表型特征（phenotypic signature）作为遗传
代理指标（genetic proxy），检测了六个属的有鳞目爬行动物（squamate reptiles）（它们均独立进化出后肢减少，且目前缺乏遗传和发育研究工具）的后肢方向不对称性：
Agamodon anguliceps、Bachia intermedia、Chalcides sepsoides、Indotyphlops
braminus、Ophisaurus attenuatuas、O. ventralis以及Teius teyou。其中一个类群Chalcides sepsoides存在显著的
不对称性：其左侧骨盆和股骨残迹分别比右侧大18%和64%，这表明该物种中
Pitx1表达发生了改变。然而，其余五个类群要么表现为右侧更大的不对称性，要么无
方向不对称性，这表明有鳞目爬行动物及更广泛的脊椎动物中，后肢减少存在多种发育遗传通路。