鲨鱼是最早进化生命的动物之一。如今，约有70％的鲨鱼生下了活着的年轻人，其余的产卵。
最初，所有发育中的鲨鱼胚胎都由胎儿蛋黄-SAC滋养。对于产卵物种，当婴儿鲨鱼开始独立进食时，这种蛋黄足以看到胚胎到孵化。
In live birthing species however, mothers supply additional nutrition once the yolk-sac has been depleted. The exact form this additional maternal care takes varies considerably from species to species.
Some species produce a mucus in the uterus for the growing embryos to feed on, while in others a supply of unfertilised eggs is provided for the embryos to eat. Many shark species use the more familiar (to we mammals) strategy of direct nutrient exchange between mother and fetus via a placenta.
Requiem for a placenta
人们认为胎盘发育仅在鲨鱼中进化一次，仅在270种地鲨（Carcharhiniformes）中发现。
按此顺序的家庭之一是Requiem Sharks（Carcharhinidae），该组织包含许多熟悉的物种，例如Blacktip和Bull Sharks。所有60种气氛鲨鱼都有胎盘，除了一个：虎鲨。反而，as recent researchhas shown, tiger shark mothers provide nutrients for their developing embryos by filling the egg case with an energy rich fluid.
由于虎鲨的最亲近的亲戚是胎盘，因此其祖先很可能确实有一个胎盘，后来在进化过程中丢失了。生殖策略的这种巨大转变是很少见的。一旦一个物种进化出胎盘繁殖，很少有病例知道其后代失去了这种能力。
因此，老虎鲨提供了一个独特的机会，可以查看与胎盘丢失相关的遗传变化。这是一支由研究人员领导的团队Vincent Savolainen在伦敦帝国学院，研究本周出版inBMC Evolutionary Biology。
Chasing the tracks in their genes
The team took muscle samples from one individual tiger shark and individuals from five placental requiem shark species, all sampled off the coast of the Bahamas. They then sequenced the transcriptomes of these 6 species, along with transcriptomes of two species from different shark families (one placental, one non-placental) to act as genetic outgroups for the comparisons among the requiem sharks.
By constructing a phylogenetic tree of the relationships among the species, the researchers were able to look for evidence of positive selection on certain tiger shark genes. Differences in the transcriptome of tiger sharks compared to their placental relatives implies these genes were associated with the loss of the placenta.
Their results found evidence for positive selection linked to placental loss in genes in two areas: sexual reproduction and brain development.
Selection on sexual reproduction
Researchers detected signatures of positive selection on the genes TCTEX1D2 and VAMP4, both known to be involved in sperm production. The protein coded by TCTEX1D2 is required for cilia function and is found in the flagellum of sperm in many taxa, including humans, mice and teleost fish. Deletion of VAMP4, meanwhile, has been linked to sperm abnormalities in mice.
鉴于这些基因已知，研究人员推测其在虎鲨中表达的变化可能意味着胎盘的损失导致了性行为的变化。
With a gestation period of 15-16 months (substantially longer than their placental relatives), and a year of sexual inactivity following birth, female tiger sharks only breed around一次过y 3 years。相反，雄性虎鲨每年繁殖。这意味着在繁殖过程中，男性之间的竞争将会发生重大竞争。女性可能会繁殖多个男性，因此这场竞争很可能会在精子竞争后进行。
过去的研究表明，老虎鲨的精子比其他鲨鱼的精子较短。较短的精子通常意味着男性可以产生更多的精子，这是精子竞争的好处。因此，我们可以推测TCTEX1D2和VAMP4的变化反映了雄性虎鲨的精子计数的增加（随后较短的精子），以适应精子竞争的增加，这本身是由于女性育种模式的变化而引起胎盘。
大脑发育的选择
Signatures of positive selection were found in two genes linked to brain development, ARL6IP5 and YWHAE.
The brain is one of the most energetically expensive organs to develop and for this reason non-placental sharks are expected to have relatively smaller brains. Notably then, the YWHAE gene has been linked to brain development in humans, mice and drosophila. Potentially, changes in this gene are due to the tiger shark adapting to the reduction in energy available to the developing embryo due to the loss of the placenta.
The other gene, ARL6IP5, encodes a transmembrane protein known to inhibit the transport of glutamate through the brain. When deprived of oxygen, glutamate levels increase, causing neuronal death and potentially brain damage.
老虎鲨鱼具有几乎全球的分布和利用各种栖息地和食物来源的能力，通常会发现自己在低氧环境中。对于大多数鲨鱼物种，这种栖息地会导致缺乏氧气到达大脑。那么，似乎有可能的是，ARL6IP5表达的增加是一种适应，使虎鲨能够在追捕猎物时利用贫民窟的环境。
过去将来会更加清晰
As the researchers are quick to point out, these results are only preliminary. Future sequencing of DNA from a greater variety of sharks (in particular comparisons with the few other shark species known to have secondarily lost placentas), and more individuals from each species, will of course improve our ability to identify genetic footprints caused by loss of the placenta.
直到最近，几乎没有对基因组水平的鲨鱼进行的研究，这主要是由于其基因组的庞大大小。与斑马鱼的1.4 GB相比，每个单倍体基因组超过30 GB。但是，随着DNA测序变得越来越便宜，我们可能会看到越来越多的数据为这类研究收集。
For now, we can be content with these tantalising glimpses of the genetic changes and evolutionary adaptations caused by this dramatic and unusual evolutionary loss.
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