You might think flowers don’t have much choice about who they mate with, given they are rooted to the ground and can’t move.

But when scientists from Nagoya, Japan used powerful microscopes to study the fertilization process, they were surprised to find the female part of a flowering plant (ovules) could repel sperm from pollen and direct them to nearby unfertilized ovules in the same plant.

First though, it’s important to understand how reproduction in flowering plants works. Just like animals, flowering plants engage in sexual reproduction where male and female parts come together and create new life.

In both flowering plants and animals, these reproductive cells, also known as gametes, contain half the number of chromosomes found in normal adult cells. The fusion of gametes restores the normal number of chromosomes and allows the development of an embryo that can eventually develop into an adult, like the plants and people you see around you.

Most organisms produce far more sperm than eggs. In mammal reproduction, the sperm are highly mobile, and many arrive at the egg around the same time. Yet multiple fertilization rarely happens. This would introduce unneeded chromosomes, unbalance the embryo’s genome and probably lead to developmental abnormalities including death.

Flowering plants face similar challenges in matching one sperm to one egg, but they handle it quite differently from mammals like us. Even the production of eggs and sperm in plants is more complex.

Pollen, which carries the male gametes, is produced in specialized organs called anthers. These are the oval shaped parts forming the top of the stamen. When the anthers rupture, which needs to be synchronized with flower development, mature pollen grains are exposed. These pollen grains are transferred to the female parts of the flower, often through the help of wind, insects, birds, or other pollinators. But numerous biological gatekeepers, or barriers, ensure that only appropriate pairings happen.

When the pollen arrives on the sticky receptive surface of the female part of the flower, called the stigma, which is part of the pistil, the pollen has to germinate on the stigma. It then grows down through the style, towards the egg, which resides deep inside the ovule. The pollen can only do this if it is compatible with the pistil. Just like in animals, reproducing within the family can have disadvantages in plants, such as poor growth.

To avoid these issues, around 50% of flowering plant species have developed a mechanism called self-incompatibility, which helps to prevent inbreeding. For instance, when pollen and pistil proteins recognize each other as being from the same plant, a signal is sent to block the growth of the pollen tube, preventing fertilization.

But many pollen grains can land on a stigma and germinate. So, how do plants ensure that each ovule is only penetrated by just one pollen tube? Using live cell microscopy along with special fluorescent trackers, scientists can observe and measure changes inside cells. This technology helps us understand how pollen tube growth is controlled by monitoring different aspects of cell activity, such as energy levels, acidity and cellular structures.

The recent study from Japan used advanced imaging techniques to show that protein signals guide a pollen tube to an individual ovule within the ovary, through a process called chemotaxis. Chemotaxis acts a bit like a navigation system where the growing tip of the pollen tube homes in on the source of these protein signals.

The system also ensures that each ovule pairs with just one pollen tube. The researchers found the system includes a repulsion signal too. Once a pollen tube is fixed on a particular ovule, a different signal prevents additional pollen tubes from approaching that same ovule and redirects pollen tubes to other ovules.

This precise orchestration ensures successful fertilization and efficient seed production, which is essential for producing our food.

There’s another barrier when the pollen tube releases the sperm cells into the ovule. Most non-flowering, often referred to as “lower,” plants such as ferns, mosses and algae, have mobile male gametes that are similar to animal sperm. The sperm of flowering plants, however, have lost their mobility and are delivered to their destinations by the pollen tube which can grow at speeds of up to 1cm per hour.

Throughout its journey within the female parts of the flower (the stigma, style and ovule), intense communication happens between the pollen tube and the various parts of the pistil. The ovule secretes attractants, small proteins called LUREs, which guide pollen tubes to grow towards it. Once the tube reaches the ovule, it enters and releases its two sperm cells.

In a fascinating evolutionary twist, these two sperm perform a double fertilization: one sperm fertilizes the egg cell while the other fertilizes a special cell called the central cell. The fertilized egg cell develops into the embryo that will grow into a new plant, while the fertilized central cell creates an endosperm. The endosperm is a kind of tissue that supports and feeds the embryo, much like the mammalian placenta feeds the unborn baby.

Although the endosperm is temporary in many species and the seed is primarily just embryo, in grasses, the endosperm forms a large part of the ripe seed that we harvest for making foods like bread, rice and porridge.

Plants are so different from us it is easy to dismiss them as simple. But every year scientists are learning more about how intricate and complex their lives are.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

According to a study by researchers at William & Mary’s Batten School of Coastal & Marine Sciences, the American lobster may be more resilient to the effects of climate change than expected. For the first time, experiments performed at the Virginia Institute of Marine Science (VIMS) have documented how female American lobsters groom their offspring, providing evidence that these behaviors are not significantly impacted by temperature and acidity levels forecasted for Maine’s coastal waters by the end of the century.

The findings are published in the journal Marine Ecology Progress Series.

Despite being one of the largest commercial fisheries in the U.S. with an annual economic impact of more than $460 million in Maine alone, few studies have documented the reproductive behavior of female American lobsters. With the Gulf of Maine warming faster than nearly any other ocean surface on the planet, it’s important to understand how the effects of climate change will impact the sustainability of the species and the fishery it supports.

“Brood grooming by female lobsters has been anecdotally observed, but it had not been quantitatively recorded before,” said Abigail Sisti, who is completing her Ph.D. in Marine Science at the Batten School and is lead author on the study. “In other crustaceans, these behaviors can have a significant impact on the survival of their offspring. Because the environment supporting the lobster fishery is rapidly changing, we wanted to understand how it might impact the way they care for their offspring.”

Female American lobsters can produce thousands of eggs that they hold under their tails for long periods of time, between five to 12 months, as the embryos develop. In other crustaceans, grooming behaviors help clear out parasites, remove dead eggs and facilitate the flow of water carrying oxygen and nutrients through the densely packed egg masses.

The study was part of a larger effort to determine how multiple stressors affect the reproductive success of the species. In this study, the researchers were testing whether increases in water temperatures and acidity had an impact on grooming behaviors and embryo survival.

“The long-term nature of this experiment required somewhat of a moonshot approach,” said Rivest, whose seawater aquarium laboratory at VIMS has been specifically designed to control multiple environmental variables over long periods of time. “The conditions of our control group were set to match current conditions in the Gulf of Maine, while our experimental groups corresponded to temperature and pH predictions for the end of the century.”

In addition to their research outcomes, Sisti and others produced an educational curriculum to involve students and teachers in the research. The research as well as the lesson plans are documented through a story map produced by the National Oceanic and Atmospheric Administration’s Sea Grant and Ocean Acidification Program.

Documenting grooming behavior

The researchers partnered with officials from Maine’s Department of Marine Resources to secure lobsters from commercial operations for use in the study. They obtained female lobsters at a marketable size with intact legs, which are frequently lost in the wild or when they are caught. In total, they observed the behavior of 24 lobsters for five months, or until the embryos matured. Sisti and other students had the daunting task of reviewing dozens of hours of recordings from underwater cameras and documenting the frequency and type of grooming behaviors as well as the overall survival of the embryos.

Lobsters in experimental groups experienced temperature increases of 4 degrees Celsius and acidification levels of -0.5 pH from present conditions. Oxygen levels remained constant for all animals to isolate the effects of temperature and acidity.

Rivest, Sisti and others expected to observe several different grooming behaviors, and they hypothesized that grooming would increase in response to environmental stressors.

“We did observe a number of grooming behaviors that increased in frequency during embryo development,” said Sisti. “However, neither water temperature nor acidification at the levels in our experiments caused significant behavioral changes or impacted embryo survival. This is encouraging because it shows lobsters may be reproductively resilient to forecasted environmental changes.”

Three distinct grooming behaviors were observed by the researchers:

• Tail fanning, where the lobsters elongate and retract their tails. This promotes water movement throughout the eggs.
• Pleopod fanning, where the lobsters use small fins called pleopods on their tails to circulate water around and through the egg mass.
• Pereopod probing, where lobsters use one of their five pereopods, or walking legs, to poke and probe the eggs. This is thought to help remove parasites and dead eggs while jostling the embryos, although excessive probing can damage embryos or completely strip the egg mass.

Despite the optimistic outcomes, questions remain

The researchers are optimistic about the results of the study for the future of the fishery. However, they caution there are still many other variables to consider.

“While this study focused primarily on maternal behavior, future studies will explore other factors like how the embryos themselves are affected over time by different stressors,” said Rivest. “We also want to explore how these factors are impacting the overall well-being of egg-bearing female lobsters as well as conditions that exist in microenvironments frequented by lobsters.”

The variables in the study were determined using forecasts for open ocean conditions. However, wild lobsters often prefer specific habitats like rock crevices and sand mounds that provide cover from predators. There, the conditions may differ dramatically from those in open water.

“In our experiments, control temperatures were akin to spring and summertime conditions in the Gulf of Maine, which may have accelerated embryo development. We recommend future experiments explore behavioral changes during winter-time conditions,” said Sisti.

While questions remain, these are positive findings for the American lobster. However, they stand in contrast to findings in other crustacean species.

“Our results highlight the need for species-specific investigations of behavior and reproduction under climate change conditions,” said Rivest. “Further understanding of how co-occurring environmental factors influence all aspects of reproduction will be essential for predicting and managing the long-term success of the American lobster fishery.”