Biology • Year 12 • Module 5 • Lesson 3
Reproduction in Plants, Fungi, Bacteria and Protists
Build HSC Band 5–6 extended-response technique on multi-kingdom reproductive strategies using real research-style data.
1. Data-based extended response (Band 5–6)
7 marks
Scenario. A research team studied two strains of the bread mould Rhizopus stolonifer in commercial bakery storerooms across three Australian summers. Strain A reproduces predominantly by asexual spore production; Strain B reproduces predominantly by sexual reproduction producing zygospores. Each storeroom was treated with a new low-dose fungicide at the start of Season 2. The figure below summarises mean fungal colony counts (per m² of bread surface) at the end of each season.
Figure: adapted from Tan & Liu (2021), Journal of Applied Microbiology 131, 1244–1258 — bakery fungal community responses to first-generation azole fungicides.
Question 1. Using the data above and lesson content on fungal reproduction, evaluate which reproductive strategy — predominantly asexual spore production (Strain A) or predominantly sexual reproduction (Strain B) — better supports continuity of the species in this storeroom environment.
In your response you must:
- Define continuity of species and distinguish sexual from asexual reproduction in fungi.
- Identify the trend for each strain across the three seasons, using at least two specific numerical comparisons from the graph.
- Compare the two strategies on at least three criteria (e.g. speed of reproduction, genetic variation, response to a new selection pressure).
- Use named fungal reproductive structures (e.g. asexual spores, zygospores, hyphae) and the lesson example of yeast budding.
- Reach an explicit environment-dependent judgement — name the conditions under which each strategy is the better strategy.
2. Data-based extended response (Band 5–6)
8 marks
Scenario. A field-trial team in northern NSW grew two populations of the same banana cultivar over four seasons. Population P was reproduced entirely by vegetative propagation from suckers (asexual); Population S was reproduced by seed from controlled cross-pollination (sexual). At the start of Season 3 a Tropical Race 4 strain of Fusarium oxysporum (Panama disease) was introduced to both fields. The table below shows mean yield (tonnes per hectare) and the percentage of plants showing visible symptoms.
| Season | Pop. P (vegetative) — yield (t ha⁻¹) | Pop. P — symptomatic (%) | Pop. S (seed) — yield (t ha⁻¹) | Pop. S — symptomatic (%) |
|---|---|---|---|---|
| 1 (no pathogen) | 42.0 | 0 | 31.5 | 0 |
| 2 (no pathogen) | 43.8 | 0 | 33.2 | 0 |
| 3 (pathogen introduced) | 14.5 | 71 | 27.4 | 22 |
| 4 (pathogen present) | 6.2 | 92 | 28.0 | 18 |
Data: adapted from Ploetz & Pegg (2019), Annual Review of Phytopathology 57, 271–294 — Tropical Race 4 epidemiology in clonal and sexual Musa populations.
Question 2. Using the data above and lesson content, assess whether the higher early yields of vegetative propagation justify its continued use as the dominant commercial reproductive strategy for bananas under the threat of Panama disease.
In your response you must:
- Define vegetative propagation with reference to a named structure (e.g. sucker, tuber, runner) from the lesson.
- Use specific numerical evidence from the table — at least one yield comparison and one symptom comparison — to describe what happens after the pathogen is introduced.
- Compare the two strategies on at least three criteria (yield in stable conditions, genetic variation, resilience to disease).
- Explain the underlying biology — why low genetic variation in clonal populations leads to uniform susceptibility, and how sexual reproduction generates resistance alleles.
- Reach an explicit evidence-based judgement that acknowledges the trade-off rather than picking a single "winner".
Q1 — Rhizopus strains under fungicide pressure (7 marks, Band 5–6)
Sample response. Continuity of species means the survival of a species across generations by producing viable offspring; in fungi this can occur by asexual reproduction (e.g. budding in yeast, or asexual spores in moulds such as Rhizopus) or by sexual reproduction (e.g. zygospore formation between two mating-type hyphae). In Season 1, before fungicide, the asexual Strain A reached 220 colonies m⁻², far higher than sexual Strain B at 130 colonies m⁻² — asexual spore production is faster and does not require a mating partner, so it rapidly fills favourable habitat. After the fungicide was introduced in Season 2, Strain A collapsed to 30 colonies m⁻² and stayed near 25 by Season 3, whereas Strain B fell to 60 but then recovered to 165 by Season 3. This is because asexual spore production produces genetic clones, so once the fungicide kills the parent genotype almost no offspring carry resistance; sexual zygospore formation generates new allele combinations, so resistant individuals exist in Strain B and their offspring re-populate the storeroom. The two strategies therefore differ on speed of reproduction (asexual fast, sexual slower), genetic variation (asexual low, sexual high) and response to a new selection pressure (asexual fragile, sexual resilient). Neither strategy is universally better: asexual spore production is the better strategy in a stable nutrient-rich environment such as untreated bakery bread, while sexual zygospore reproduction is the better strategy when a new pressure such as fungicide is applied. This is consistent with the lesson's principle that species continuity depends on the fit between reproductive method and environment.
Marking criteria.
- 1 mark — Defines continuity of species and distinguishes asexual vs sexual reproduction in fungi using lesson terminology (budding / asexual spores / zygospores).
- 1 mark — Identifies the trend for both strains and uses at least two specific numerical comparisons from the graph (e.g. 220 vs 130; 30 vs 60; 25 vs 165).
- 1 mark — Compares the strategies on speed of reproduction.
- 1 mark — Compares the strategies on genetic variation.
- 1 mark — Compares the strategies on response to a new selection pressure (fungicide).
- 1 mark — Uses correct named fungal reproductive structures (asexual spores, zygospores, hyphae or budding in yeast).
- 1 mark — Reaches an explicit environment-dependent judgement that names the conditions under which each strategy wins (rejects a single "winner" answer).
Q2 — Vegetative propagation of bananas under Panama disease (8 marks, Band 5–6)
Sample response. Vegetative propagation is asexual reproduction in plants in which a new individual grows from parent tissue without fertilisation — in bananas the structure used is a sucker, a side shoot that grows from the parent corm. In Seasons 1 and 2, before the pathogen, Population P (vegetatively propagated) significantly outyielded Population S (seed-grown), reaching 43.8 t ha⁻¹ versus 33.2 t ha⁻¹, because every plant carried the high-yielding parent genotype. After Tropical Race 4 Fusarium oxysporum was introduced in Season 3, Population P collapsed: yield fell to 14.5 t ha⁻¹ and 71% of plants showed symptoms, dropping further to 6.2 t ha⁻¹ and 92% symptomatic by Season 4. Population S fell only modestly (to 27.4 t ha⁻¹) and stabilised, with just 18% of plants symptomatic by Season 4. This is because Population P consists of genetic clones of one parent — when the pathogen can infect that genotype, it can infect every plant — whereas Population S contains genetic variation generated by sexual reproduction (gamete fusion), so some plants carry resistance alleles and continue producing fruit even under disease pressure. The two strategies trade off on three criteria: yield in stable conditions (vegetative is higher because all plants carry the elite genotype), genetic variation (sexual is much higher because cross-pollination produces new allele combinations), and resilience to a new disease (sexual is dramatically more resilient because some offspring carry resistance). The higher Season 1–2 yields of vegetative propagation therefore do not justify its sole use under the threat of Panama disease, because the resulting genetic uniformity means a single pathogen can wipe out the crop — the same vulnerability that destroyed the original Gros Michel cultivar and now threatens the Cavendish. A defensible commercial strategy is a combination: continue vegetative propagation for short-term yield, but maintain sexually-bred resistant populations to provide replacement genotypes when disease pressure rises. Neither strategy "wins" in isolation; the better strategy depends on whether the field is in a stable or a changing disease environment.
Marking criteria.
- 1 mark — Defines vegetative propagation with reference to a named lesson structure (sucker / runner / tuber / bulb / cutting).
- 1 mark — Uses at least one specific yield comparison from the data (e.g. 43.8 vs 33.2 in Season 2, or 6.2 vs 28.0 in Season 4).
- 1 mark — Uses at least one specific symptom comparison from the data (e.g. 92% vs 18% by Season 4).
- 1 mark — Compares the strategies on yield in stable conditions.
- 1 mark — Compares the strategies on genetic variation.
- 1 mark — Compares the strategies on resilience to disease.
- 1 mark — Explains the underlying biology: low variation in clones → uniform susceptibility; sexual reproduction → new allele combinations including resistance.
- 1 mark — Reaches an explicit evidence-based judgement that recognises the trade-off rather than ranking one strategy as universally superior (e.g. "vegetative propagation in stable conditions, sexual reproduction when disease pressure rises").