Ambystoma platineum and Ambystoma texanum have been found to be tetraploid. Most Ambystoma hybrids are described by how many haploid sets of chromosomes they contain from each of their parent species, reflecting their level of ploidy. Mortality Embryonic mortality in parthenogenic amphibians is high. Hatching rates for North American salamander species have ranged from 19.5% to 30.5%. It is speculated that intergenomic exchanges, like crossing over during meiosis, may play a role. Intergenomic exchanges are often lethal due to the fact that chromosomes in unisexual species are homeologous (similar, but less so than homologous chromosomes from within a species). Homologous chromosomes are largely identical, in this case as a result of the chromosome replication. Modes of parthenogenesis and parthenogenetic-like reproduction in amphibians Gynogenesis Gynogenesis is a form of parthenogenesis where an egg begins to divide only after being pricked by a sperm cell, but without the genetic material of the sperm being used. There are two known mechanisms of gynogenesis. The first is an endomitotic event prior to meiosis, where the number of chromosomes in a cell doubles without cell division taking place. After meiosis each egg has the same ploidy (number of chromosomes) as the mother. This particular parthenogentic mechanism has been observed in unisexual Ambystoma species as well as Glandirana rugosa. The second potential mechanism is apomixis, which produces a complete set of chromosomes through mitotic replication. This method has not been observed in any amphibious species. Courtship behavior between females of the same species has been observed in Ambystoma platineum, and has been posited to induce either oviposition of ovulation, though the precise utility of the behavior is unknown. Hybridogenesis In hybridogenesis, females of a unisexual species mate with a male of a related species and utilize their genetic material in order to produce offspring. However, in spite of this requirement, the genetic material of the male is not passed on to the next generation. Just prior to meiosis, during mitotic division, spindle fibers attach to the maternal chromosomes, leaving the paternal chromosomes in the cytoplasm. The paternal chromosomes are therefore excluded from nascent eggs, without recombination having typically occurred. In some cases, such as Pelophylax esculentus, there is also endomeiosis prior to cell division, which means that the maternal chromosomes are duplicated and each egg contains identical pairs of chromosomes. Hybridogenesis can be described as a parthenogenetic-like mode of reproduction, since there is no continuing heredity