Parental Investment Strategies in Xiphophorus Fish

Sexually reproducing organisms show a wide range in how much they invest in their offspring. In some species, this investment ends before fertilization, while in others, it continues even after the young reach adulthood. For example, some fish release millions of eggs into the water to be fertilized, while large mammals, like orca whales, may care for just one young for many years.

Typically, there is a link between how much a parent invests in its young and the chances of those young surviving. However, there can be trade-offs between the energy a parent spends on their offspring and their own survival and reproduction. Factors such as how many times a parent can reproduce, how many young they have at one time, and their overall lifespan can all play a role in this balance.

#Offspring Size and Survival

One method to measure parental investment is through the size of the offspring. Generally, larger offspring have better survival chances until they are mature. Though not always the case, larger young tend to avoid dangers like being eaten or starving. Researchers have speculated that understanding the reasons for differences in offspring size among species might explain how these traits evolved.

Another way for parents to help their young survive is through viviparity, where Embryos develop internally rather than in eggs. This method can lower the risk of death for embryos and can also provide a better environment during development. Some species can even provide nutrients directly to their young during this time. Most species offer some nutrition via yolk in the egg, whereas others give nutrients both before and after fertilization.

The evolution of different strategies for providing nutrients after fertilization has happened many times throughout species. Some fish families are a great case study for examining these strategies, especially when it comes to viviparity and how it affects offspring size.

#Studying Poeciliid Fishes

The poeciliid fish family allows for an exploration of how offspring size and nutrient strategies have evolved. These fish can give birth to live young, and over time, their ancestors have developed different ways of investing in their young. Some offer no post-fertilization care, while others provide extensive support. Surprisingly, in some poeciliid species, larger offspring may not always come from mothers that provide more nutrients.

This discrepancy can be complicated because the size of the young often depends on traits related to the mother, such as her own size and age. Environmental factors can also heavily influence young size regardless of how much care the mother gives.

For instance, in low-predation environments, mothers may invest in larger eggs, resulting in bigger newborns, while in high-predation areas, the eggs may be smaller. There are many variables to consider when looking at differences in offspring size, particularly when comparing closely related species.

#The Xiphophorus Species

Xiphophorus species provide a unique way to understand these topics because they inhabit a range of ecological environments. Some live in rivers near sea level, while others thrive in higher mountainous streams. Previous studies suggested that Xiphophorus species do not provide nutrients after fertilization, leading to a focus on egg size alone. However, this idea needs further investigation, particularly in species from extreme environments.

In this research, we look at Xiphophorus malinche, which have developed remarkably large young. We aim to uncover the reasons for this size increase. By examining embryos in both controlled lab environments and in their natural habitats, we will investigate the factors that lead to greater offspring size in this fish.

#Measuring Fry Size Across Species

To compare the sizes of newborn fry from different Xiphophorus species, we collected groups of fry as they were born. Each brood was weighed and measured, and the average sizes were calculated. We looked at five species: X. birchmanni, X. malinche, X. cortezi, X. pygmaeus, and X. variatus. Notably, Xiphophorus malinche fry were significantly larger than all others.

We also examined Hybrids between X. malinche and X. birchmanni. The F1 hybrids had sizes more like X. malinche, while the F2 and natural hybrids were intermediate. An estimate of the heritability of fry size suggests a genetic component to these differences.

#Measuring Embryo Weight

To see how embryos develop over time in Xiphophorus species, we measured their dry weights at different stages. We collected pregnant females from the wild, along with a small number of non-pregnant females with yolked eggs. By analyzing the dry weights of embryos at various stages, we can compare the growth patterns of X. malinche and X. birchmanni.

Data showed that X. malinche embryos maintain their weight over development, indicating some form of nutrient provision, while X. birchmanni embryos lost weight, suggesting a lack of post-fertilization care. This difference in developmental profiles hints that X. malinche mothers provide more resources to their embryos compared to X. birchmanni.

#Embryo Size in F1 Hybrids

To directly compare embryo sizes from F1 hybrids and pure species, we artificially inseminated both X. malinche and X. birchmanni. Our results confirm that X. malinche embryos remain larger than X. birchmanni embryos. F1 hybrids, depending on which parent they come from, exhibit sizes that fall between their parents.

The presence of larger embryos in the X. malinche cross suggests that maternal effects and offspring genetics both play a role in determining size.

#Gene Expression Analysis

We also explored gene expression in the ovarian tissue of both species. Specific genes related to nutrient provisioning and pregnancy were found to be more active in X. malinche. This suggests that there may be mechanisms at play that enhance nutrient supply to developing embryos. Notably, genes associated with prolactin signaling, involved in nutrient delivery during pregnancy in mammals, were highly expressed in X. malinche compared to the other species.

Immunochemistry tests confirmed that prolactin protein is present in the ovarian tissues of both species, suggesting that both species might share this hormonal system, but X. malinche appears to use it more effectively for nourishing their young.

#Ecological Differences in Habitats

When comparing habitats, we noticed that X. malinche tend to live in cooler, less nutrient-rich waters than X. birchmanni. As a result, X. malinche produce larger offspring, likely as an adaptation to low resource availability. We also found differences in seasonal breeding, with X. malinche showing a more pronounced seasonal pattern in reproduction compared to X. birchmanni, which breeds year-round.

To test how well fry from both species tolerate starvation, we put them in conditions with and without food. Though both species grew similarly when food was plentiful, X. malinche fry showed better tolerance to starvation compared to X. birchmanni fry, suggesting that being larger at birth helps them survive harder conditions.

#Differences in Cross Viability

Finishing the research, we found that the viability of hybrid offspring varied based on the direction of the cross. X. malinche mothers and X. birchmanni fathers produced many viable F1 offspring, while the reverse pairing often resulted in premature births or stillbirths. This indicates that maternal effects significantly influence the health of the young, pointing to a clear genetic conflict over nutrient allocation during embryonic development.

#Conclusion

Through this research, we demonstrate substantial variation in offspring size within Xiphophorus species. The findings suggest that larger offspring size in X. malinche may have evolved to adapt to their environment's limited resources. Moreover, the initial signs of matrotrophy in X. malinche indicate a shift in reproductive techniques compared to closely related species.

Future studies will need to investigate how these different reproductive strategies contribute to overall fitness and survival in various ecological contexts. The unique interplay of genetic and environmental influences on offspring size is crucial to understanding the evolutionary dynamics within this fascinating group of fish.