| Viviparity is a reproductive mode that evolves from oviparity through gradual increases in the length of egg retention and intrauterine development until give birth to young. It has evolved independently in more than 100 lineages of squamate reptiles (lizards, snakes and amphisbaenians). The adaptive significance of viviparity is that, gravid females are able to adjust thermoregulation to provide more suitable or stable temperatures for their developing embryos. And viviparity offers pervasive benefits by lowering embryonic mortality, accelerating embryonic development, optimizing the offspring phenotype and enhancing offspring fitness, and obviating the need for females to find suitable egg-laying sites. These advantages make viviparous lizards can adapt to different habitat, and distribute in many extreme climate regions. The evolutionary transition from oviparity to viviparity is propelled undoubtedly by natural selection, and greatly affect the reproduction, morphology, physiology and behavior of related species. Toad-headed lizards of the genus Phrynocephalus (Agamidae), either oviparous or viviparous species, found in China are mainly distributed in cold or arid regions with large temperature differences and high heterogeneities. These organisms therefore provide ideal model systems to study the adaptation to extreme climate and the evolution of viviparity in reptiles. In this dissertation, we used Phrynocephalus lizards as model systems, focusing on the apects of reproductive mode, ecomorphology, molecular phylogeny and thermal physiology, to mainly study the adaptation to extreme climate in lizards, selective basis and adaptive significance of the evolution of viviparity, the impact of climate warming on reptiles, as well as phenotypic plasticity and fitness relevance of experimental manipulation on egg retention length. Integrated data were also gathered to discuss the problems with species classification and speciation of the genus Phrynocephalus.Ecomorphology is primarily concerned about the interaction between morphology and ecology. Lizards often show significant inter-specific variation in morphological traits that are highly sensitive to the environment, thus, provide good model systems for such studies. Significant variation in all examined mensural and meristic characters were found in seven viviparous Phrynocephalus lizards that differ in habitat use. A principal component analysis resolved two components (with eigenvalues≥1) from 19 morphological variables in females, so did in males. The first component, explaining 40.3% variance in females and 38.2% variance explained in males, had high positive loadings for nasal scales, supraocular scales, dorsal scales, ventral scales, scales around mid-body, subdigital lamellae of the 4th finger and subdigital lamellae of the 4th toe in both sexes. The second component (15.3% variance explained in females and 15.9% variance explained in males) had a high negative loading for hind-limb length in males. The seven species differed in their scores on the first and second axes, and females and males both could be roughly divided into five groups based on their scores on the first two axes:[PG], [PF], [PP], [PE, PT, PZ], [PV]. The cluster analyses generally support such a division. Bayesian analyses based on mitochondrial ND4 or 16S rDNA sequences strongly support Phrynocephalus monophyly and monophyly of the viviparous group with strong posterior probability. The relationship between P. zetangensis and P. theobaldi is identical with previous reports, P. zetangensis was believed to be included in P. theobaldi, and the two species were also very similar in terms of morphological traits. Phrynocephalus guinanensis was phylogenetically closely related to P. putjatia, but the cluster analyses on morphological traits showed that the two species had extremely significant difference.The distributional regions of viviparous and oviparous groups of the genus Phrynocephalus are significantly different in climate, altitude and other environmental factors. We used 10 species of toad-headed lizards (three oviparous and seven viviparous species) as model systems, to mainly test differences in morphological and reproductive strategies between lizards of different reproductive modes. The main difference of the two groups was that they had different patterns of sexual dimorphism in body size:adult females and males were monomorphic in body size in P. guinanensis, adult females were the larger sex in the other six viviparous toad-headed lizards, and adult males were the larger sex in three oviparous species. Furthermore, ventral ground color of tail tip was different between the two groups. One-way ANOVA revealed that females were longer in abdomen length but shorter in tail length in all species of the same SVL; females had smaller heads than did males of the same SVL in most species. Postpartum body mass, litter size, neonate or egg mass, litter or clutch mass and relative litter mass differed among the ten Phrynocephalus lizards. Reproductive output was correlated positively with abdomen length, but negatively with head length. Head width did not affect reproductive output, nor did tail length. Based on the cluster analysis for five female reproductive variables, the ten species were not be divided into distinct viviparous or oviparous group, but could be roughly divided into five groups:[PG], [PF], [PE, PP, PV], [PT, PZ, PPG] and [PA, PGR], with P. guinanensis being much more different from the remaining species.Temperature is one of the most important environmental factors, and can significantly affect life-history traits of reptiles, such as birth date, reproductive output and offspring phenotype. Pregnant females of P. vlangalii were divided into five groups with different thermal conditions (29℃,32℃,35℃, TR and NAT). Thermal conditions experienced by females affected birth date, neonate mass and reproductive output but not postpartum body mass and litter size in P. vlangalii. Female size (SVL) and neonate mass in the NAT treatment were significantly larger than those in the other groups, and with larger litter mass than those in the constant temperature treatment. Birth date was significantly different among the five groups, from earlier to later following the sequence:35℃,32℃, 29℃, TR and NAT. Thermal regimes experienced by females affected the morphological traits of P. vlangalii neonates. A principal component analysis resolved two components (with eigenvalues≥1) from 6 morphological variables in neonates, accounting for 59.6% of variation. The first component, explaining 37.2% variance, had high a positive loading for fore-limb length. Neonates could be roughly divided into three groups based on their positions on the first two axes:[NAT], [TR,35], [29,32]. Three viviparous lizards from Qinghai were grouped by using 2 (background temperatures,18℃and 22℃)×2 (thermoregulation opportunities, daily thermoregulating for 15 h and 9 h) factorial design experiment. Significant variation in all examined reproductive variables was found in the three species. Both thermoregulation opportunity and background temperature affected birth date but not other female reproductive traits examined. Three-way ANOVA revealed that neonates of the three species had significant inter-specific variation in morphological traits. Thermoregulation opportunities did not affect the morphological traits of neonates. Background temperature significantly affected body mass, abdomen length, head width, tail length and fore-limb length of neonates. Neonates from 18℃background temperature were larger in body mass, head width, tail length and fore-limb length, but smaller in abdomen length than those from 22℃background temperature. A principal component analysis resolved two components (with eigenvalues≥1) from 7 morphological variables in neonates, accounting for 62% of variation. The first component (43.7% variance explained) had high a negative loading for head width, and had high positive loadings for tail length, fore-limb length and hind-limb length. The second component (18.3% variance explained) had a high negative loading for head length.Phrynocephalus putjatia (viviparous) and P. przewalskii (oviparous) were used as the model systems to test the possible differential impacts of climate warming on reptiles of different reproductive modes, by using 2 (species)×3 (background temperatures) factorial design experiemnt. Three-way ANOVA (with background temperature, species and sex as the factors) revealed that:lizards at lower background temperatures thermoregulated more actively than did those at higher background temperatures; viviparous lizards thermoregulated more actively than did oviparous lizards; at none of the three background temperatures did thermoregulatory activities differ between the sexes. Lizards in the 28℃treatment selected higher body temperatures than did those in the 24℃treatment, and lizards in the 24℃treatment selected higher body temperatures than did lizards in the 20℃treatment. Gravid females selected lower body temperatures than did nongravid females and adult males, and nongravid females and adult males did not differ in selected body temperature. The two Phrynocephalus lizards did not differ in selected body temperature. Neither in P. putjatia nor in P. przewalskii did mean values for SVL, postpartum body mass, litter size, litter mass and relative litter mass differed among females under the three thermal regimes. Background temperature affected birth date in P. putjatia but not in P. przewalskii. Female P. putjatia gave birth earliest in the 28℃treatment and latest in the 20℃treatment, with the 24℃treatment in between. Female P. przewalskii in the 24℃treatment laid larger eggs than did females of the same SVL in the other two treatments. Neither in P. putjatia nor in P. przewalskii did background temperature affect any hatchling trait examined.We experimentally manipulated the length of egg retention in an oviparous species (P. przewalskii) to find the clues for the evolutionary transition from oviparity to viviparity. One-way ANOVA analysis showed that the length of egg retention significantly affected embryonic stage at oviposition. The mean embryonic stage was earliest in the oxytocin-injected females and latest in the desiccation-exposed females, with control females in between. Incubation length was negatively correlated with embryonic stage at oviposition. Linear regression showed that the mean incubation length was shortest in eggs at stages 26-29 and longest in eggs at stage 31-33, with eggs at stage 30 in between. Embryonic stage at oviposition affected egg mass but not all hatchling traits examined, with eggs at stages 26-29 on average being smaller than those at later embryonic stages. fitness relevance of egg retention length. |
PDF Full Text Request