Data from: Mortality and morphology in egg masses of unisexual and Jefferson Salamanders

Unisexual Ambystoma salamander egg masses have often been observed to exhibit very high rates of embryo mortality. The ecological consequences and underlying mechanisms are of great concern to researchers and managers studying these and other members of the species complex, all of which are listed as rare species throughout much of their range. Substantial embryo mortality is commonly used by field ecologists as an indicator that unisexual salamanders are present in a pond; egg masses of unisexual salamanders appear otherwise very similar to those of A. jeffersonianum (Jefferson Salamander). Early researchers suggested that elevated mortality among unisexual salamanders was due to lack of fertilization caused by sperm limitation. However, recent work has suggested that embryo failure is due to genetic errors particular to the unisexual salamander lineage. Our goals in this study were to (1) identify when during development embryonic mortality occurs in unisexual salamanders, and (2) to develop a morphological metric to distinguish egg masses of unisexual and Jefferson salamanders. Collecting from sites across western Massachusetts, we reared 356 eggs from 11 egg masses of known species identity in the laboratory, examined field photographs of 96 egg masses of known species identity (based on mitochondrial sequencing and 6 microsatellite alleles), and examined 757 field photographs of egg masses of unknown species identity. We developed a simple, scale-independent metric to distinguish Jefferson Salamander egg masses from those of co-occurring unisexual salamanders. Among developing embryos beyond the earliest stages, we found no difference in mortality rates between unisexual salamanders and Jefferson Salamanders. However, we observed a large pulse of embryo mortality in the earliest stages of development, followed by a trickle of additional mortality at later stages. Our results suggest that the primary cause of embryo mortality in Massachusetts populations of unisexual salamanders involves failure of embryos to initiate development.

单性钝口螈（Unisexual Ambystoma Salamander）的卵团常被观测到表现出极高的胚胎死亡率。研究这类及该物种复合群（species complex）其他成员的研究者与管理者始终高度关注其生态后果与潜在机制——该物种复合群的所有类群在其多数分布区内均被列为稀有物种。野外生态学家通常将显著的胚胎死亡率作为某池塘存在单性钝口螈的指示标志：单性钝口螈的卵团外观与杰斐逊钝口螈（A. jeffersonianum, Jefferson Salamander）的卵团极为相似，难以仅凭外观直接区分。早期研究者认为，单性钝口螈的胚胎死亡率升高源于精子限制（sperm limitation）引发的受精失败。但近期研究表明，胚胎死亡实则是单性钝口螈谱系特有的遗传错误所致。本研究的目标为：（1）明确单性钝口螈的胚胎死亡发生在发育的哪个阶段；（2）构建可区分单性钝口螈与杰斐逊钝口螈卵团的形态学指标（morphological metric）。研究团队从马萨诸塞州西部的多个采样点采集样本，在实验室中培育了11个已知物种身份的卵团中的356枚卵；通过线粒体测序（mitochondrial sequencing）与6个微卫星等位基因（microsatellite alleles）鉴定物种身份后，分析了96个已知物种身份的卵团的野外照片；同时分析了757个物种身份未知的卵团的野外照片。本研究构建了一种简单的、与尺度无关的形态学指标，可有效区分同域分布的杰斐逊钝口螈与单性钝口螈的卵团。在发育早期之后的胚胎阶段，单性钝口螈与杰斐逊钝口螈的胚胎死亡率并无显著差异。但我们观测到，在发育最早期存在一次大规模的胚胎死亡脉冲，后续发育阶段则仅有少量零星的额外死亡。本研究结果表明，马萨诸塞州种群中单性钝口螈的胚胎死亡主要源于胚胎无法启动发育过程。