Evolution of Carpel: Angiosperms Study Notes

One of the major evolutionary innovations of angiosperms is the carpel (Scutt et al., 2006).
- The carpel is defined as the female part of the flower and serves as a key characteristic of angiosperms.
- Vialette-Guiraud and Scutt (2009) described it as the female reproductive organ that encloses the ovules in flowering plants.
- This contrasts with gymnosperms, where ovules are not enclosed by any structure, hence referred to as “naked” or exposed.
- The importance of carpels lies in their role within the angiosperm flower and in the broader diversification of flowering plants.
Characteristics of Carpels:
- The carpel is the basic unit of the angiosperm female reproductive structure.
- The term carpel is often used interchangeably with pistil, although a pistil may consist of two or more carpels.
- The carpel or pistil (often referred to as the gynoecium) is one of four whorls that constitute the angiosperm flower.

The four whorls of the flower are:
1. Calyx: Green sepals.
2. Corolla: Brightly colored petals.
3. Androecium: The male structures called stamen.
4. Gynoecium: Female structures including carpels.
The calyx and corolla collectively form the perianth.
All four whorls are supported by a structure known as the receptacle, which is attached to a stalk called the peduncle.
Morphological variations among different species include:
- Presence or absence of the four whorls.
- Types of symmetry: complete/incomplete, actinomorphic/zygomorphic, perfect/imperfect, epigynous/hypogynous.
- Floral arrangements: solitary flowers vs. clusters (inflorescence).
Variations are significant for plant identification and may include additional features that attract pollinators (accessory floral structures and semaphylls).

Angiosperms have developed various modifications in flowers to attract pollinators, which contributes to diversity and dominance in terrestrial ecosystems.
These attractants are categorized into two groups:
1. Accessory Floral Structures:
- Composed of: involucre, corona, and nectaries.
- Example for the involucre: Observe any available flower to understand its function.
- Corona: A complex structure associated with petals, often seen in the Apocynaceae family, which provides a landing platform for pollinators.
- Nectaries: Specialized structures that secrete nectar (sugar-containing compounds). Nectaries can be either floral or extrafloral.
1. Semaphore Structures (semaphylls):
- Leaf-like structures that attract insects or animals seeking nectar and pollen.
- Types include intrafloral semaphylls (from the flower) and extrafloral (from other plant parts like stems and leaves).
Co-evolution of flowers and pollinators is discussed, noting varied adaptations that facilitate successful reproduction like foul odors (e.g., Rafflesia) to attract flies, and vibrant colors for bees (Aster) or nocturnal moths (Hoya).

The APG has transformed the classification system for angiosperms by developing an ordinal classification primarily based on DNA sequencing.
APG updates:
- APG I (1998) established the initial classification.
- Subsequent updates occurred in 2003, 2009, and 2016, providing clarity to taxonomists and systematists.
Current classification splits angiosperms into four major clades:
1. Basal Angiosperms and Magnoliids
2. Monocots
3. Rosids
4. Asterids

Microsporogenesis (male spore formation):
- Occurs in the anther, part of the third whorl composed of two lobes with pollen sacs.
- Inside each pollen sac, sporogenous tissue forms microspore mother cells (microsporocytes), nourished by the tapetum.
- Microspore mother cells undergo meiosis, resulting in a tetrad of haploid microspores, which mature into pollen grains.
Microgametogenesis (male gamete formation):
- Observed in pollen grain slides, which contain a tube nucleus and generative nucleus.
- During pollen tube elongation, the tube nucleus guides the tube towards the ovule's micropyle; the generative nucleus mitotically divides into two sperm nuclei.
Megasporogenesis (female spore formation):
- Involves examination of the ovary of Lilium, noting structures like the funiculus, integuments, micropyle, and nucellus.
- The nucellus gives rise to a large megasporocyte, which undergoes meiosis to produce four megaspores, and typically one survives as the functional megaspore.
Megagametogenesis (female gamete formation):
- This process begins with the functional megaspore undergoing three mitotic divisions to form an embryo sac (female gametophyte), which eventually contains eight nuclei with specific fates:
  - Three antipodals at the chalazal end.
  - Two polar nuclei at the center.
  - Two synergids and one egg cell at the micropyle.

Basal Angiosperms and Magnoliids:
- Represent primitive flowering plants, often called ANITA grade.
- Includes orders like Amborellales, Nymphaeales, and Austrobaileyales, with several key species (e.g., magnolias, avocados).
- Occupy a lower tier in the phylogenetic tree, indicating their primitive status.
Monocots:
- Characteristics include parallel venation, scattered vascular bundles (atactostelic stele), and generally single cotyledons.
- Example representatives include orchids and economically important crops such as rice and corn.
Eudicots (true dicotyledons):
- Refers to a significant group of angiosperms described by distinct characteristics including pollen structure.
- Further divided into specific classes like Rosids, Caryophyllids, and Asterids.
Rosids:
- Known for their extensive floral diversity and include both Fabids and Malvids.
Caryophyllids:
- Includes Caryophyllales, characterized by unique traits distinct from other clades.
Asterids:
- A large group known for sympetalous corollas and distinct floral arrangements, with many economically significant species.