Reproductive assurance

Reproductive assurance (fertility assurance) occurs as plants have mechanisms to assure full seed set through selfing when outcross pollen is limiting. It is assumed that self-pollination is beneficial, in spite of potential fitness costs, when there is insufficient pollinator services or outcross pollen from other individuals to accomplish full seed set.. This phenomenon has been observed since the 19th century, when Darwin observed that self-pollination was common in some plants.[1] Constant pollen limitation may cause the evolution of automatic selfing, also known as autogamy. This occurs in plants such as weeds, and is a form of reproductive assurance.[2]  As plants pursue reproductive assurance through self-fertilization, there is an increase in homozygosity , and inbreeding depression, due to genetic load, which results in reduced fitness of selfed offspring.[3] Solely outcrossing plants may not be successful colonizers of new regions due to lack of other plants to outcross with, so colonizing species are expected to have mechanisms of reproductive assurance - an idea first proposed by Herbert Baker and referred to as Baker's Law.[4] Baker’s Law predicts that reproductive assurance should be common in weedy plants that persist by colonizing new sites.[5] As plants evolve towards increase self-fertilization, energy is redirected to seed production rather than characteristics that increased outcrossing, such as floral attractants, which is a condition known as the selfing syndrome.[2]

Evolution

Reproductive assurance is thought to be a driver for the evolution of selfing because it would promote purging of genetic load [4][2] and it contributes to the occurrence of mixed mating systems. There are a number of mechanisms that result in reproductive assurance, but delayed selfing has been the one most studied. When pollination is unsuccessful, full seed set can be obtained through delayed selfing. Most hermaphrodite plants are self-compatible, meaning they are able to self-fertilize. When pollinators routinely fail to deliver adequate outcross pollen to ensure reproduction, selfing may increase through mechanisms of reproductive assurance, leading to the evolution of complete selfing.[2]

Mechanisms

Mechanisms of reproductive assurance include:

Delayed selfing

A common reproductive assurance mechanism that occurs in plants that are able to reproduce by self-fertilization by changing the position of the anthers and stigma within the flower to promote self-pollination.

Cryptic self-incompatibility (CSI)

Cryptic self-incompatibility favors fertilization by outcrossing pollen, when both outcross and self-pollen are present on the same stigma.[6] CSI promotes fertilization by outcross pollen due to faster growth rate of outcross pollen tubes. Reproduction assurance occurs when there is insufficient outcross pollen present to attain fertilization of all of the ovules.

Autogamy

Similar to delayed selfing, fertilization via autogamy occurs when there is a lack of pollinators and has evolved as a form of reproductive assurance to ensure successful reproduction.[7]

Cleistogamy

Cleistogamous flowers are produced along with chasmogamous flowers on the same plant resulting in a mixed mating system that ensures reproductive success through autogamy.[8]

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References
  1. Darwin C (2009). The Effects of Cross and Self Fertilisation in the Vegetable Kingdom. Cambridge: Cambridge University Press. doi:10.1017/cbo9780511694202.001. ISBN 978-0-511-69420-2.
  2. Lloyd DG (1979). "Some Reproductive Factors Affecting the Selection of Self-Fertilization in Plants". The American Naturalist. 113 (1): 67–79. doi:10.1086/283365.
  3. Busch JW, Delph LF (February 2012). "The relative importance of reproductive assurance and automatic selection as hypotheses for the evolution of self-fertilization". Annals of Botany. 109 (3): 553–62. doi:10.1093/aob/mcr219. PMC 3278291. PMID 21937484.
  4. Evans ME, Hearn DJ, Theiss KE, Cranston K, Holsinger KE, Donoghue MJ (July 2011). "Extreme environments select for reproductive assurance: evidence from evening primroses (Oenothera)". The New Phytologist. 191 (2): 555–63. doi:10.1111/j.1469-8137.2011.03697.x. PMID 21449951.
  5. Cheptou PO (February 2012). "Clarifying Baker's Law". Annals of Botany. 109 (3): 633–41. doi:10.1093/aob/mcr127. PMC 3278284. PMID 21685434.
  6. Kruszewski LJ, Galloway LF (2006). "Explaining Outcrossing Rate in Campanulastrum americanum (Campanulaceae): Geitonogamy and Cryptic Self‐Incompatibility". International Journal of Plant Sciences. 167 (3): 455–461. doi:10.1086/501051. ISSN 1058-5893.
  7. Asande LK, Omwoyo RO, Oduor RO, Nyaboga E (2020-05-12). "A simple and fast Agrobacterium-mediated transformation system for hybrid passion fruit (Passiflora edulis f. edulis × Passiflora edulis f. flavicarpa)". Plant Methods. doi:10.21203/rs.3.rs-27621/v1.
  8. Veena V, Nampy S (2019). "Induced cleistogamy: A strategy for reproductive assurance in Murdannia nudiflora (Commelinaceae)". Botany. 97 (10): 547–557. doi:10.1139/cjb-2019-0007. ISSN 1916-2790.
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