Collaborative research: Is hybridization among threatened Caribbean coral species the key to their survival or the harbinger of their extinction?

环礁珊瑚构成了整个加勒比海浅海热带珊瑚群落的基础。然而，自20世纪80年代以来，曾经占主导地位的鹿角珊瑚(Acropora Cervicorris)和鹿角珊瑚(A.palmata)已经减少了90%以上，主要是由于疾病。珊瑚礁的持续减少危及珊瑚礁提供大量社会和生态效益的能力，包括来自海鲜捕捞和旅游业的经济收入，以及保护海岸线免受风暴和飓风造成的极端海浪事件的影响。尽管自2006年以来受到美国《濒危物种法案》的保护，但造礁珊瑚的生存威胁仍然普遍存在，包括疾病和海洋温度变暖，这可能导致进一步的大规模死亡。然而，这些密切相关物种之间的杂交正在增加，并可能为适应不断变化的环境提供一条途径。虽然杂交种在过去很少见，但它们现在在极端温度和光照的浅水生境中茁壮成长，并正在向亲本物种的栖息地扩张。更多的证据表明，杂交种比至少一个亲本物种具有更强的抗病能力。因此，杂交可能有可能通过与两个亲本物种交配的杂交种转移适应基因来拯救受威胁的亲本物种免于灭绝，但广泛的基因流动可能会改变亲本物种的进化轨迹，导致一个或两个物种灭绝。这个合作项目是为了收集遗传和生态数据，以了解杂交物种丰度增加的机制。从这项研究中获得的知识将有助于在当前和新出现的对珊瑚生存的威胁下对珊瑚种群进行更具战略性的管理。该项目包括为高中、本科生和研究生以及博士后研究员提供的综合研究和教育机会。美属维尔京群岛的学生将参加珊瑚产卵研究，资源管理人员将接受有关顶孔繁殖的培训，以应用珊瑚恢复技术。目前的模型预测，由于海洋表面温度上升和海洋酸化，珊瑚礁将在未来200年内消亡。因此，必须确定栖息地、分类群和进化机制，以使一些珊瑚物种能够保持其作为基础动物的作用。杂交可以为适应不断变化的条件提供一条途径。珊瑚会以一定的频率进行杂交，结果可能会从通过渐渗将一些等位基因引入现有的亲本物种，到诞生一个新的、或许更适应环境的遗传谱系。唯一被广泛接受的珊瑚杂交系统由曾经占主导地位但现在受到威胁的加勒比海物种--颈角珊瑚和棕榈珊瑚组成。在过去，由麋鹿和鹿角珊瑚之间的自然杂交产生的杂交群体很少见，杂交繁殖的证据仅限于与鹿角珊瑚罕见的交配。最近的实地观察表明，杂交种正在增加，其生态作用正在整个加勒比海地区发生变化。这些杂交种似乎较少受到这种疾病的影响，这种疾病导致了近几十年来其亲本物种的大规模死亡。杂交种也被发现在高温和光照的浅层栖息地茁壮成长，这表明它们可能不太容易受到未来变暖情景的影响。与此同时，它们正在向更深层次的亲代物种栖息地扩张。初步的遗传数据表明，杂交种现在正在相互交配，这表明了形成新物种的可能性。此外，杂交种似乎能够与鹿角珊瑚和鹿角珊瑚交配，可能通过杂交种在亲本物种之间进行基因流动。有人建议进行研究，以解决杂交的增加以及随后的导入将如何影响加勒比顶孔动物目前的生态角色和未来的进化轨迹。具体地说，这个合作项目旨在回答以下问题：1)跨物种范围和基因组的历史渗入速度、方向和程度是什么？基于三个重复杂交区的全基因组SNP基因分型的连锁区块分析将回答这个问题。2)下一代混合编队目前的规模和未来的潜力是什么？形态计量学和遗传分析将与体外受精试验相结合。3)是什么机制让杂交生物在炎热的浅水中茁壮成长？一系列操纵性的原位和非原位实验将确定生物或非生物因素是否有利于在浅水中杂交生存。4)杂交种是否比亲本种更具抗病能力？将进行相互移植实验中的疾病传播分析和组织学分析，以确定疾病的程度。将采取多学科方法，结合传统和尖端技术，对加勒比珊瑚的进化生态进行详细分析。

Reef-building acroporid corals form the foundation of shallow tropical coral communities throughout the Caribbean. Yet, the once dominant staghorn coral (Acropora cervicornis) and the elkhorn coral (A. palmata) have decreased by more than 90% since the 1980s, primarily from disease. Their continuing decline jeopardizes the ability of coral reefs to provide numerous societal and ecological benefits, including economic revenue from seafood harvesting and tourism and shoreline protection from extreme wave events caused by storms and hurricanes. Despite their protection under the U.S. Endangered Species Act since 2006, threats to the survival of reef-building acroporid corals remain pervasive and include disease and warming ocean temperatures that may lead to further large-scale mortality. However, hybridization among these closely related species is increasing and may provide an avenue for adaptation to a changing environment. While hybrids were rare in the past, they are now thriving in shallow habitats with extreme temperatures and irradiance and are expanding into the parental species habitats. Additional evidence suggests that the hybrid is more disease resistant than at least one of the parental species. Hybridization may therefore have the potential to rescue the threatened parental species from extinction through the transfer of adapted genes via hybrids mating with both parental species, but extensive gene flow may alter the evolutionary trajectory of the parental species and drive one or both to extinction. This collaborative project is to collect genetic and ecological data in order to understand the mechanisms underlying increasing hybrid abundance. The knowledge gained from this research will help facilitate more strategic management of coral populations under current and emerging threats to their survival. This project includes integrated research and educational opportunities for high school, undergraduate and graduate students, and a postdoctoral researcher. Students in the United States Virgin Islands will take part in coral spawning research and resource managers will receive training on acroporid reproduction to apply to coral restoration techniques. Current models predict the demise of reefs in the next 200 years due to increasing sea surface temperatures and ocean acidification. It is thus essential to identify habitats, taxa and evolutionary mechanisms that will allow some coral species to maintain their role as foundation fauna. Hybridization can provide an avenue for adaptation to changing conditions. Corals hybridize with some frequency and results may range from the introduction of a few alleles into existing parent species via introgression, to the birth of a new, perhaps better adapted genetic lineage. The only widely accepted coral hybrid system consists of the once dominant but now threatened Caribbean species, Acropora cervicornis and A. palmata. In the past, hybrid colonies originating from natural crosses between elkhorn and staghorn corals were rare, and evidence of hybrid reproduction was limited to infrequent matings with the staghorn coral. Recent field observations suggest that the hybrid is increasing and its ecological role is changing throughout the Caribbean. These hybrids appear to be less affected by the disease that led to the mass mortality of their parental species in recent decades. Hybrids are also found thriving in shallow habitats with high temperatures and irradiance suggesting they may be less susceptible to future warming scenarios. At the same time, they are expanding into the deeper parental species habitats. Preliminary genetic data indicate that hybrids are now mating with each other, demonstrating the potential for the formation of a new species. Further, hybrids appear to be capable of mating with both staghorn and elkhorn coral, perhaps leading to gene flow between the parent species via the hybrid. Research is proposed to address how the increase in hybridization and perhaps subsequent introgression will affect the current ecological role and the future evolutionary trajectory of Caribbean acroporids. Specifically, this collaborative project aims to answer the following questions: 1) What is the historic rate, direction, and degree of introgression across species ranges and genomes? Linkage block analysis based on genome-wide SNP genotyping across three replicate hybrid zones will answer this question. 2) What is the current extent and future potential of later generation hybrid formation? Morphometric and genetic analyses combined with in vitro fertilization assays will be used. 3) What mechanisms allow hybrids to thrive in hot, shallow waters? A series of manipulative in situ and ex situ experiments will determine whether biotic or abiotic factors favor hybrid survival in shallow waters. 4) Are hybrids more disease resistant than the parentals species? Disease transmission assays in reciprocal transplant experiments and histological analysis to determine the extent of disease will be conducted. A multidisciplinary approach will be taken that combines traditional and cutting edge technology to provide a detailed analysis of the evolutionary ecology of Caribbean corals.