The abutment of egg and agent occurs at fertilization; this creates a zygote that will advance into a new, altered animal accustomed a admixture of the affectionate genomes. Thus, animal reproduction, involving two abstracted gametes, diversifies the abiogenetic basin and accelerates evolution. However, the added call for fertilization poses challenges. Aboriginal and foremost, specific matchmaking is critical. Agent charge accommodated egg, and absolutely one agent needs to beget absolutely one egg because an unfertilized egg cannot develop, and fertilization with too abounding agent (polyspermy) is appropriately fatal. Furthermore, fertilization has to accommodate a barrier amid breed by creating egg-sperm incompatibilities. Despite the alluring biological questions, evolutionary implications, and bright applications for animal contraception and infertility inherent in compassionate the mechanisms by which egg and agent meet, these are still ailing understood. On folio 1029 of this issue, Herberg et al. (1) investigate fertilization in zebrafish and analyze Bouncer, a protein that is important for sperm-egg alternation and additionally creates a species-specific barrier. This analysis takes us a footfall afterpiece to compassionate species-specific fertilization and breed evolution.
Bouncer is a receptor that is bidding on the alfresco apparent of the egg film area it selects agent from the aforementioned species, preventing fertilization by agent from added species.
Fertilization is a awful orchestrated accident (2). In preparation, both the egg and agent charge change: The agent increases its advancement and the agent arch is reorganized to anatomy a cap-like structure, alleged the acrosome. This prepares the agent for assimilation of the egg covering (the specialized extracellular cast that surrounds the egg), attachment, and closing admixture with the egg membrane. Egg activation leads to changes in the egg covering (also alleged the zona pellucida or chorion) (3). In mammals, the egg covering not alone is a structural basic that protects the oocyte but additionally attracts and binds sperm, admitting in fish, the micropyle, a specific aperture in the egg coat, forms the distinct access point for sperm. The egg covering becomes beneath absorbent aloft fertilization to anticipate polyspermy. The fertilization action was aboriginal declared in sea urchins by Oscar Hertwig in 1876 (4). Since then, abounding apparatus of the agent and egg accouterment accept been active in assorted aspects of the process. Surprisingly few of these accept withstood the analysis of gene abatement abstracts to authenticate an capital action for egg-sperm acceptance and fusion, aegis adjoin polyspermy, or breed specificity (2).
The discoveries of the sperm-specific agency Izumo (5) and its egg-specific receptor Juno (6) appear the aboriginal brace of factors appropriate for fertilization in mammals. Izumo, called afterwards a Japanese altar committed to the Shinto celestial of marriage, encodes a single-pass transmembrane protein that binds via its N-terminal Izumo area to Juno, called afterwards the Greek goddess of marriage, which belongs to the folate receptor ancestors and is anchored to the egg film via a GPI (glycosylphosphatidylinositol) linker (7, 8). Beyond egg-sperm recognition, their exact role in fertilization has not been established. For example, although the brace mediates adherence amid agent and egg or amid heterologous cells, it is not acceptable to advance the admixture of two membranes in vitro (6). It has been proposed that vesicles cogent Juno on their membranes are extruded from the oocyte to action as an oocyte allurement to anticipate polyspermy (6), possibly with the tetraspanin ancestors protein CD9, but the apparatus is alien (9). Izumo and Juno homologs abide throughout mammals, and their action in egg-sperm acceptance could advice discriminate adjoin agent from added species. However, affirmation suggests that this receptor brace is not complex in the breed barrier (5, 10).
Searching for oocytespecific announcement of genes with alien action in zebrafish (11), Herberg et al. apparent Bouncer, a short, GPI-linked film protein and affiliate of the Ly6/urokinase plasminogen activator receptor family. Bouncer-deficient zebrafish are viable, but about all females are absolutely sterile. The authors begin no accessible defects in egg morphology, and egg activation seemed to advance normally. Agent accumulated at the micropyle, but the agent basis bootless to access the egg back Bouncer was absent. The authors removed the egg covering and empiric that agent did not bind calmly to Bouncer-deficient eggs.
To investigate whether Bouncer is complex in same-species recognition, Herberg et al. activated whether Bouncer can analyze agent from altered species. They articular a homolog of Bouncer from a distantly accompanying fish, medaka, which shares alone 40% arrangement character with zebrafish-Bouncer. Unlike zebrafish-Bouncer, medaka-Bouncer did not restore abundance to Bouncer-deficient zebrafish back fertilized with zebrafish sperm. However, zebrafish eggs cogent medaka-Bouncer and fertilized with medaka agent acquired some abundance (5.7% of eggs from abundant females produced embryos, compared with a abundance amount beyond 78% back wild-type zebrafish were fertilized with zebrafish sperm). The consistent embryos were hybrids that agitated a affectionate zebrafish genome and a benevolent medaka genome. The amalgam embryos survived aboriginal embryogenesis, underwent gastrulation, and formed an anteroposterior axis, but bootless to advance accomplished 48 hours. Thus, Bouncer not alone is all-important for species-specific eggsperm recognition, but is additionally acceptable to accommodate this action in the attendance of agent from addition breed (see the figure). The low fertilization amount back application medaka-Bouncer and medaka agent is conceivably not surprising, accustomed that all added apparatus were zebrafish-specific and acceptable contributed to optimal recognition.
The abutting affinity to Bouncer in mammals is agent acrosome membrane-associated protein 4 (SPACA4, additionally accepted as SAMP14), a GPI-linked protein that is bidding by agent and anticipation to accessory with the acrosome (12). Curiously, there seems to be sex-specific announcement of Bouncer homologs according to the approach of fertilization (1). In fishes and amphibians, eggs are fertilized externally, so egg-sperm interactions are the aboriginal barrier for breed recognition. In mammals, fertilization occurs internally with agent traveling through the changeable changeable tract. Alternation of agent with actual beef forth the way to the oocyte has been anticipation to accommodate some breed barrier (2). Conceivably sperm-expressed Bouncer is a species-specific cue that is accustomed by actual beef above-mentioned to egg encounter. Clearly, assuming of phenotypes associated with SPACA4 absence will accommodate new insight. Additionally, advertent the analogue or counterparts that are apprenticed by Bouncer on the agent of fishes, and on the eggs and possibly actual beef lining changeable tracts in mammals, will advice us to accept this puzzle.
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