Heger Peter, Kroiher Michael, Ndifon Nsah, Schierenberg Einhard
Zoological Institute, University of Cologne, Köln, Germany.
BMC Dev Biol. 2010 May 17;10:51. doi: 10.1186/1471-213X-10-51.
MAP (mitogen-activated protein) kinase activation is a prerequisite for oocyte maturation, ovulation and fertilisation in many animals. In the hermaphroditic nematode Caenorhabditis elegans, an MSP (major sperm protein) dependent pathway is utilised for MAP kinase activation and successive oocyte maturation with extracellular MSP released from sperm acting as activator. How oocyte-to-embryo transition is triggered in parthenogenetic nematode species that lack sperm, is not known.
We investigated two key elements of oocyte-to-embryo transition, MSP expression and MAP kinase signaling, in two parthenogenetic nematodes and their close hermaphroditic relatives. While activated MAP kinase is present in all analysed nematodes irrespective of the reproductive mode, MSP expression differs. In contrast to hermaphroditic or bisexual species, we do not find MSP expression at the protein level in parthenogenetic nematodes. However, genomic sequence analysis indicates that functional MSP genes are present in several parthenogenetic species.
We present three alternative interpretations to explain our findings. (1) MSP has lost its function as a trigger of MAP kinase activation and is not expressed in parthenogenetic nematodes. Activation of the MAP kinase pathway is achieved by another, unknown mechanism. Functional MSP genes are required for occasionally emerging males found in some parthenogenetic species. (2) Because of long-term disadvantages, parthenogenesis is of recent origin. MSP genes remained intact during this short interval although they are useless. As in the first scenario, an unknown mechanism is responsible for MAP kinase activation. (3) The molecular machinery regulating oocyte-to-embryo transition in parthenogenetic nematodes is conserved with respect to C. elegans, thus requiring intact MSP genes. However, MSP expression has been shifted to non-sperm cells and is reduced below the detection limits, but is still sufficient to trigger MAP kinase activation and embryogenesis.
丝裂原活化蛋白（MAP）激酶的激活是许多动物卵母细胞成熟、排卵和受精的先决条件。在雌雄同体的线虫秀丽隐杆线虫中，一种依赖主要精子蛋白（MSP）的途径用于MAP激酶的激活以及随后的卵母细胞成熟，从精子中释放的细胞外MSP作为激活剂。在缺乏精子的孤雌生殖线虫物种中，卵母细胞向胚胎的转变是如何触发的尚不清楚。
我们研究了两种孤雌生殖线虫及其近缘雌雄同体亲属中卵母细胞向胚胎转变的两个关键要素，即MSP表达和MAP激酶信号传导。尽管无论生殖方式如何，所有分析的线虫中都存在激活的MAP激酶，但MSP表达有所不同。与雌雄同体或双性物种不同，我们在孤雌生殖线虫中未在蛋白质水平上发现MSP表达。然而，基因组序列分析表明，几个孤雌生殖物种中存在功能性MSP基因。
我们提出了三种不同的解释来阐释我们的发现。（1）MSP已失去其作为MAP激酶激活触发因子的功能，且在孤雌生殖线虫中不表达。MAP激酶途径的激活是通过另一种未知机制实现的。功能性MSP基因对于一些孤雌生殖物种中偶尔出现的雄性是必需的。（2）由于长期存在的劣势，孤雌生殖是最近才出现的。尽管MSP基因无用，但在这个短时间间隔内它们仍保持完整。与第一种情况一样，一种未知机制负责MAP激酶的激活。（3）在孤雌生殖线虫中调节卵母细胞向胚胎转变的分子机制与秀丽隐杆线虫是保守的，因此需要完整的MSP基因。然而，MSP表达已转移到非精子细胞，且降低到检测限以下，但仍足以触发MAP激酶激活和胚胎发生。
